mahalakshmi engineering college-trichy...
TRANSCRIPT
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 1
UNIT -V
PART-A
1Define skin depth (or)Depth of penetration (AU NOVDEC10)
It is defined as the wave has been attenuated to 37 of its original value is known
as skin depth
1
In a good conductor the rate of attenuation is very high and the wave penetrate only at a
very short distance before being reduced to negligible small percentage of its original
strengthThis term is known as skin depth
2Define skin effect (AU NOVDEC11)
The inductance of the inner region of the conductor is greater than the outer regionsince
flux linkages of the inner region is higher than the outer region So most of the current is
confined to outer region This is known as skin effect
3Define Polarisation (AU NOVDEC12)
The polarization of the uniform plane wave refers to the time varying behaviour of the
electric field strength vector at some fixed point in space
It can be classified into three typesThey are
(i)Linear polarization
(ii)Circular polarisation
(iii)Electrical polarisation
4Define propagation constant (AU APR 09)
The propagation constant can be expressed in terms of properties of the medium as
( )j j j
5Define Standing wave ratio (AU MAY 11)
It is the ratio of the maximum to minimum magnitude of voltage or current on a line
having standing wave ratio
S= max
min
1
1
KV
V K
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 2
where K is the reflection coefficient
6What is Brewster angle (AU APRMAY 09)
Brewster angle is an incident angle at which there is no reflected wave for parallelly
polarized wave
1 2
1
tan
7State the poynting theorem (AU NOVDEC100911)
The vector product of the electric field intensity and the magnetic field intensity at any
point is a measure of the rate of energy flow at that point
P=EtimesH
8Mention the two properties of the uniform plane wave (AU NOVDEC11)
(i) At every point in space the electric field E and magnetic field H are perpendicular to each
other and to the direction of the travel
(ii)The fields vary harmonically with time and at the same frequency everywhere in space
9Write the equation of skin depth of conductor (AU NOVDEC09)
Skin depth 1
f
10Write the wave equation in conducting medium (AU NOVDEC08)
2
2
20
E HE
t t
22
20
E HH
t t
11What is meant by charteristic impedance (AU NOVDEC08)
It is the ratio of electric field to the magnetic field or it is the ratio of square root of
permeability to permittivity to the medium
η=EH
12What is the velocity of the electromagnetic wave in free space and in lossless
dielectric (AU NOVDEC 07)
The velocity in lossless dielectric is
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 3
Vp =1 =R R
C
PART-B
1Deduce the equation of the propagation of of the plane electromagnetic waves in free space ( AU NOVDEC 12)
The wave equation for a plane wave traveling in the x direction is
where v is the phase velocity of the wave and y represents the variable which is changing as the wave passes This is the form of the wave equation which applies to a stretched string or a plane electromagnetic wave The mathematical description of a wave makes use of partial derivatives
In two dimensions the wave equation takes the form
which could describe a wave on a stretched membrane
General Wave Equations
Electromagnetic (EM) wave can be ultimately described by three things 1) Maxwell equations material response to the EM fields and boundary conditions
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 4
HMEPt
DJH
EuJt
BE
HMHHBB
PEEEDD
me
v
m
rv
00
00
00
10
In charge free medium (v=0) linear isotopic ( is scalar) homogeneous and time invariant medium (P responds to E instantaneously) We have
t
EEH
t
HE
H
E
0
0
if the EM wave only contains one frequency component we can rewrite the Maxwell equation in phasor form using E = Re (Es e
jωt)
sss
ss
s
s
EjEH
HjE
H
E
0
0
Use the third equation (Faradayrsquos Law) and apply the curl on both sides of the full time-varying
equation
Ht
E
Use the last equation into the above equation and use a vector identity
2
22
t
E
t
EEE
t
EE
tE
Since we are considering a charge-free region the general wave equation (Helmholtz equation)
becomes
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 5
2
22
t
E
t
EE
Again if the E and H fields only have one frequency components we can get wirte a phasor
equation for the time-harmonic field
or ss EE
22 where the material properties which govern the wave propagation are
described by
jjj
Where characterizes materials absorption (gain) and characterize the propagation speed
(propagation constant)
Uniform plane waveguide (UPW) solution
Now letrsquos try to solve the waveguide equation The simplest solution beyond trial (0) is the UPW
solution While we have
a) Electrical filed only have one components (say Ex component)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 2
where K is the reflection coefficient
6What is Brewster angle (AU APRMAY 09)
Brewster angle is an incident angle at which there is no reflected wave for parallelly
polarized wave
1 2
1
tan
7State the poynting theorem (AU NOVDEC100911)
The vector product of the electric field intensity and the magnetic field intensity at any
point is a measure of the rate of energy flow at that point
P=EtimesH
8Mention the two properties of the uniform plane wave (AU NOVDEC11)
(i) At every point in space the electric field E and magnetic field H are perpendicular to each
other and to the direction of the travel
(ii)The fields vary harmonically with time and at the same frequency everywhere in space
9Write the equation of skin depth of conductor (AU NOVDEC09)
Skin depth 1
f
10Write the wave equation in conducting medium (AU NOVDEC08)
2
2
20
E HE
t t
22
20
E HH
t t
11What is meant by charteristic impedance (AU NOVDEC08)
It is the ratio of electric field to the magnetic field or it is the ratio of square root of
permeability to permittivity to the medium
η=EH
12What is the velocity of the electromagnetic wave in free space and in lossless
dielectric (AU NOVDEC 07)
The velocity in lossless dielectric is
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 3
Vp =1 =R R
C
PART-B
1Deduce the equation of the propagation of of the plane electromagnetic waves in free space ( AU NOVDEC 12)
The wave equation for a plane wave traveling in the x direction is
where v is the phase velocity of the wave and y represents the variable which is changing as the wave passes This is the form of the wave equation which applies to a stretched string or a plane electromagnetic wave The mathematical description of a wave makes use of partial derivatives
In two dimensions the wave equation takes the form
which could describe a wave on a stretched membrane
General Wave Equations
Electromagnetic (EM) wave can be ultimately described by three things 1) Maxwell equations material response to the EM fields and boundary conditions
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 4
HMEPt
DJH
EuJt
BE
HMHHBB
PEEEDD
me
v
m
rv
00
00
00
10
In charge free medium (v=0) linear isotopic ( is scalar) homogeneous and time invariant medium (P responds to E instantaneously) We have
t
EEH
t
HE
H
E
0
0
if the EM wave only contains one frequency component we can rewrite the Maxwell equation in phasor form using E = Re (Es e
jωt)
sss
ss
s
s
EjEH
HjE
H
E
0
0
Use the third equation (Faradayrsquos Law) and apply the curl on both sides of the full time-varying
equation
Ht
E
Use the last equation into the above equation and use a vector identity
2
22
t
E
t
EEE
t
EE
tE
Since we are considering a charge-free region the general wave equation (Helmholtz equation)
becomes
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 5
2
22
t
E
t
EE
Again if the E and H fields only have one frequency components we can get wirte a phasor
equation for the time-harmonic field
or ss EE
22 where the material properties which govern the wave propagation are
described by
jjj
Where characterizes materials absorption (gain) and characterize the propagation speed
(propagation constant)
Uniform plane waveguide (UPW) solution
Now letrsquos try to solve the waveguide equation The simplest solution beyond trial (0) is the UPW
solution While we have
a) Electrical filed only have one components (say Ex component)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 3
Vp =1 =R R
C
PART-B
1Deduce the equation of the propagation of of the plane electromagnetic waves in free space ( AU NOVDEC 12)
The wave equation for a plane wave traveling in the x direction is
where v is the phase velocity of the wave and y represents the variable which is changing as the wave passes This is the form of the wave equation which applies to a stretched string or a plane electromagnetic wave The mathematical description of a wave makes use of partial derivatives
In two dimensions the wave equation takes the form
which could describe a wave on a stretched membrane
General Wave Equations
Electromagnetic (EM) wave can be ultimately described by three things 1) Maxwell equations material response to the EM fields and boundary conditions
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 4
HMEPt
DJH
EuJt
BE
HMHHBB
PEEEDD
me
v
m
rv
00
00
00
10
In charge free medium (v=0) linear isotopic ( is scalar) homogeneous and time invariant medium (P responds to E instantaneously) We have
t
EEH
t
HE
H
E
0
0
if the EM wave only contains one frequency component we can rewrite the Maxwell equation in phasor form using E = Re (Es e
jωt)
sss
ss
s
s
EjEH
HjE
H
E
0
0
Use the third equation (Faradayrsquos Law) and apply the curl on both sides of the full time-varying
equation
Ht
E
Use the last equation into the above equation and use a vector identity
2
22
t
E
t
EEE
t
EE
tE
Since we are considering a charge-free region the general wave equation (Helmholtz equation)
becomes
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 5
2
22
t
E
t
EE
Again if the E and H fields only have one frequency components we can get wirte a phasor
equation for the time-harmonic field
or ss EE
22 where the material properties which govern the wave propagation are
described by
jjj
Where characterizes materials absorption (gain) and characterize the propagation speed
(propagation constant)
Uniform plane waveguide (UPW) solution
Now letrsquos try to solve the waveguide equation The simplest solution beyond trial (0) is the UPW
solution While we have
a) Electrical filed only have one components (say Ex component)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 4
HMEPt
DJH
EuJt
BE
HMHHBB
PEEEDD
me
v
m
rv
00
00
00
10
In charge free medium (v=0) linear isotopic ( is scalar) homogeneous and time invariant medium (P responds to E instantaneously) We have
t
EEH
t
HE
H
E
0
0
if the EM wave only contains one frequency component we can rewrite the Maxwell equation in phasor form using E = Re (Es e
jωt)
sss
ss
s
s
EjEH
HjE
H
E
0
0
Use the third equation (Faradayrsquos Law) and apply the curl on both sides of the full time-varying
equation
Ht
E
Use the last equation into the above equation and use a vector identity
2
22
t
E
t
EEE
t
EE
tE
Since we are considering a charge-free region the general wave equation (Helmholtz equation)
becomes
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 5
2
22
t
E
t
EE
Again if the E and H fields only have one frequency components we can get wirte a phasor
equation for the time-harmonic field
or ss EE
22 where the material properties which govern the wave propagation are
described by
jjj
Where characterizes materials absorption (gain) and characterize the propagation speed
(propagation constant)
Uniform plane waveguide (UPW) solution
Now letrsquos try to solve the waveguide equation The simplest solution beyond trial (0) is the UPW
solution While we have
a) Electrical filed only have one components (say Ex component)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 5
2
22
t
E
t
EE
Again if the E and H fields only have one frequency components we can get wirte a phasor
equation for the time-harmonic field
or ss EE
22 where the material properties which govern the wave propagation are
described by
jjj
Where characterizes materials absorption (gain) and characterize the propagation speed
(propagation constant)
Uniform plane waveguide (UPW) solution
Now letrsquos try to solve the waveguide equation The simplest solution beyond trial (0) is the UPW
solution While we have
a) Electrical filed only have one components (say Ex component)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 6
b) This components is constant in two directions (say x-y plane) only change along z-axis
(propagation direction)
So the proposed solution is (For notational convenience we drop the subscript s)
xx azEE
How do we know this is a solution Just plug in the wave equation if we can find a Ex(x) which
satisfies the wave equation we have a solution
02
2
2
xx E
dz
Ed
The solution is
x
zz
x aeEeEzE
00
If we translate this solution back into a fully time-varying solution it is
x
ztjzztjz
x aeeEeeEzE
00
Re
The first term characterizes the forward-propagating UPW the second term characterizes the
backward-propagating one
By the help of the Maxwell equations we can also figure out other field such D B and H
ss HjE
Therefore the H field can be calculated as
ss Ej
H
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 7
(The subcript s is written here to remind you that all quanatities are phasor quantitites) Having
E and H solution available the ratio between the E and H field can be taken Letrsquos take a
forward-propagation UPW as examples
j
jj
H
E
Since E and H fields have unit (Vm) and (Am) The ratio yields a unit of () This ratio is
referred as intrinsic impedance of the materials
The wave propagates along az we can use a vector to represent that
za
The unit vector to represent this direction is defined as ap (az in this case) E-field propagates
along ax and E-field propagates along ay Therefore these three vectors form a right-handed
rectangle coordinate system ie
sps
sps
HaE
EaH
1
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 8
Now we can see the propagation characteristics for UPW is ultimately determined by
jjj We then discuss the responses in various materials
a) Propagation in lossless charge-free region
In a charge free region with zero loss the propagation constant
jjj
=0 so jjj 0
1
]coscos[
Re
00
00
p
xx
x
ztjztj
x
u
ztEztEazE
aeEeEzE
A special example will be vacuum where =0 =0
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 9
smup 103
120
8
0
0
b) Propagation in lossy media
In a lossy media the loss comes from two parts a non-zero electric conductivity 0) a
polarization loss (energy required of the filed to flip reluctant dipoles dielectric loss) A complex
permittivity c is used to characterize this part
jc
Therefore the propagation constant can be written
jjjj
jjjjj
eff
c
Where eff we can solve for the absorption and the propagation constant of the
medium ieand = Re =Im
1
12
1
12
2
2
eff
eff
Loss Tangent a standard measure of lossiness in a dielectric is measured by the ratio of
tan
eff
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 10
Referred as loss-tangent
Low-loss dielectrics
For low loss dielectrics ltlt1 rdquoltltrsquo therefore
1
eff
2
2Describe the Skin effect in detail (AU MAYJUN 10)
The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core That is the electric current tends to flow at the skin of the conductor
The skin effect causes the effective resistance of the conductor to increase with the frequency of the current The skin effect has practical consequences in the design of radio-frequency and microwave circuits and to some extent in AC electrical power transmission and distribution systems Also it is of considerable importance when designing discharge tube circuits
The current density J in an infinitely thick plane conductor decreases exponentially with depth δ from the surface as follows
where d is a constant called the skin depth This is defined as the depth below the surface of the conductor at which the current density decays to 1e (about 037) of the current density at the surface (JS) It can be calculated as follows
where
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 11
ρ = resistivity of conductor
ω = angular frequency of current = 2π times frequency
μ = absolute magnetic permeability of conductor where μ0 is the
permeability of free space (4πtimes10minus7 NA2) and μr is the relative permeability of the
conductor
The resistance of a flat slab (much thicker than d) to alternating current is exactly equal to the resistance of a plate of thickness d to direct current For long cylindrical conductors such as wires with diameter D large compared to d the resistance is approximately that of a hollow tube with wall thickness d carrying direct current That is the AC resistance is approximately
where
L = length of conductor
D = diameter of conductor
The final approximation above is accurate if D gtgt d
A convenient formula (attributed to FE Terman) for the diameter DW of a wire of circular cross-section whose resistance will increase by 10 at frequency f is
The increase in ac resistance described above is accurate only for an isolated wire For a wire close to other wires eg in a cable or a coil the ac resistance is also affected by proximity effect which often causes a much more severe increase in ac resistance
3Derive the Poynting Theorem and Poynting Vector in Electromagnetic Fields
(AU NOVDEC 09101112)
Electromagnetic waves can transport energy from one point to another point The
electric and magnetic field intensities associated with a travelling electromagnetic wave can be
related to the rate of such energy transfer
Let us consider Maxwells Curl Equations
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 12
Using vector identity
the above curl equations we can write
(1)
In simple medium where and are constant we can write
and
Applying Divergence theorem we can write
(2)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 13
The term represents the rate of change of energy stored in the electric
and magnetic fields and the term represents the power dissipation within the volume
Hence right hand side of the equation (636) represents the total decrease in power within the
volume under consideration
The left hand side of equation (636) can be written as where
(Wmt2) is called the Poynting vector and it represents the power density vector associated with the
electromagnetic field The integration of the Poynting vector over any closed surface gives the net
power flowing out of the surface Equation (636) is referred to as Poynting theorem and it states that
the net power flowing out of a given volume is equal to the time rate of decrease in the energy stored
within the volume minus the conduction losses
4In a lossless medium for which =60πmicror = 1and H=-01 cos (wt -z)ax +05 sin (wt -z)ay
AmCalculate εr (AU NOVDEC 11)
Answer
Given
=60πmicror = 1
H=-01 cos (wt -z)ax +05 sin (wt -z)ay
For lossless medium case
=0α=0β=1
To find
εr =
2
120
2
c
(i)Relative Permittivity(εr )
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 14
εr =
2
120
=22
εr =4
(ii)Angular Frequency ( )
2
c
C= Velocity of light =3times108 msec
81(3 10 )
2
=15times108 radsec
5A signal is a lossy dielectric medium has a loss tangent 02 at 550 kHZThe dielectric constant of a medium is 25Determine attenuation constant and phase constant
(AU NOVDEC11)
Given
Frequency f=550 kHz
loss tangent =02 tan θ=02
εr =25
To Find
Attenuation constant (α) Phase constant (β)
loss tangent for dielectric
tan
02
For given dielectric
1therefore it is practical dielectric
002 02 (2 )( )rf
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 15
9
3 1002 02 (2 550 10 )( 25)
36
5153 10 Sm
Attenuation constant(α)
For Practical dielectric
2
0
02
r
r
5 7
9
153 10 4 10 1
10225
36
α=182times10-3 NPm
Propagation Constant
β=
=(2πf) 0 0r r
β= 00182 radm
6A lossy dielectric is characterised by εr =25micror=4 =10-3 mhom at 10 MHzLet E=10 e-yz ax
Vmfind (i)α(ii)β(iii) (iv)Vp (v)η (AU NOVDEC 10)
Given
εr =25micror=4 =10-3 mhomf=10times106 Hz
To Find
Attenuation constant(α) Phase constant (β) Wave length Velocity of propagation(Vp) Intrinsic Impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 16
Solution
Attenuation constant(α) and Phase constant(β)
The Propagation constant for lossy dielectric
(j j
j
0 0(2 f)( )( (2 )r rj j f
6 7 3 6 12(2 10 10 )(4 10 4)(10 (2 10 10 )(8854 10 25j j )
=3 3(31582)[10 13907 10 ]j j
07355 7214 02255 07j
j
α=02255 Npm
β=07 radm
Wavelength
2
=2π07
8975m
Velocity of Propagation(Vp)
pV
2 f
=2πtimes10times106 07=8976times107 ms
Intrinsic impedance(η)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 17
j
j
0
0
(2 )
(2 )
r
r
j f
j f
=
6 7
3 6 12
(2 10 10 ) 4 10 4
10 (2 10 10 )8854 10 25
j
(12)
0
3
31582 90
17219 10 5428
η=4294 179
η=4086+j132Ω
7Explain the Polarisation of plane wave in detail (AU MAYJUN 0809)
The polarisation of a plane wave can be defined as the orientation of the electric field vector as
a function of time at a fixed point in space For an electromagnetic wave the specification of the
orientation of the electric field is sufficent as the magnetic field components are related to
electric field vector by the Maxwells equations
Let us consider a plane wave travelling in the +z direction The wave has both Ex and Ey
components
(51)
The corresponding magnetic fields are given by
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 18
Depending upon the values of Eox and Eoy we can have several possibilities
1 If Eoy = 0 then the wave is linearly polarised in the x-direction
2 If Eoy = 0 then the wave is linearly polarised in the y-direction
3 If Eox and Eoy are both real (or complex with equal phase) once again we get a linearly
polarised wave with the axis of polarisation inclined at an angle with respect to the x-
axis This is shown in fig 51
Fig 51 Linear Polarisation
4 If Eox and Eoy are complex with different phase angles will not point to a single spatial
direction This is explained as follows Let
Then
and (52)
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 19
To keep the things simple let us consider a =0 and Further let us study the nature of the electric field on the z =0 plain
From equation we find that
(53)
and the electric field vector at z = 0 can be written as
(54)
Assuming the plot of for various values of t is hown in figure 52
Figure 52 Plot of E(ot)
From equation and figure we observe that the tip of the arrow representing electric field vector
traces qn ellipse and the field is said to be elliptically polarised
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 20
Figure 53 Polarisation ellipse
The polarization ellipse shown in figure 66 is defined by its axial ratio(MN the ratio of
semimajor to semi minor axis) tilt angle (orientation with respect to x-axis) and sense of rotation(ie CW or CCW)
Linear polarization can be treated as a special case of elliptical polarization for which the axial ratio is infinite
In our example if from equation the tip of the arrow representing electric field vector traces out a circle Such a case is referred to as Circular Polarization For circular polarization the axial ratio is unity
Figure 54 Circular Polarisation (RHCP)
Further the circular polarization is aside to be right handed circular polarisation (RHCP) if the electric field vector rotates in the direction of the fingers of the right hand when the thumb points in the direction of propagation-(same and CCW) If the electric field vector rotates in the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 21
opposite direction the polarization is asid to be left hand circular polarization (LHCP) (same as CW)
In AM radio broadcast the radiated electromagnetic wave is linearly polarised with the field vertical to the ground( vertical polarization) where as TV signals are horizontally polarised waves FM broadcast is usually carried out using circularly polarized waves
In radio communication different information signals can be transmitted at the same frequency at orthogonal polarisation ( one signal as vertically polarized other horizontally polarised or one as RHCP while the other as LHCP) to increase capacity Otherwise same signal can be transmitted at orthogonal polarization to obtain diversity gain to improve reliability of transmission
8Explain the Angle Of Incidence and Brewster angle ( AU MAYJUN 11)
Angle of incidence is a measure of deviation of something from straight on for example in the
approach of a ray to a surface or the direction of an airfoil with respect to the direction of an
airplane
Optics
In geometric optics the angle of incidence is the angle between a ray incident on a surface and
the line perpendicular to the surface at the point of incidence called the normal The ray can be
formed by any wave optical acoustic microwave X-ray and so on In the figure above the red
line representing a ray makes an angle θ with the normal (dotted line) The angle of incidence at
which light is first totally internally reflected is known as the critical angle The angle of
reflection and angle of refraction are other angles related to beams
Grazing angle
When dealing with a beam that is nearly parallel to a surface it is sometimes more useful to refer
to the angle between the beam and the surface rather than that between the beam and the surface
normal in other words 90deg minus the angle of incidence This angle is called a glancing angle or
grazing angle Incidence at small grazing angle is called grazing incidence
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 22
Grazing incidence is used in X-ray spectroscopy and atom optics where significant reflection
can be achieved only at small values of the grazing angle Ridged mirrors are designed for
reflection of atoms coming at small grazing angle This angle is usually measured in milliradians
Brewsters angle
An illustration of the polarization of light which is incident on an interface at Brewsters angle
Brewsters angle (also known as the polarization angle) is an optical phenomenon named after
the Scottish physicist Sir David Brewster (1781ndash1868)
When light moves between two media of differing refractive index generally some of it is
reflected at the boundary At one particular angle of incidence however light with one particular
polarization cannot be reflected This angle of incidence is Brewsters angle θB The polarization
that cannot be reflected at this angle is the polarization for which the electric field of the light
waves lies in the same plane as the incident ray and the surface normal (ie the plane of
incidence) Light with this polarization is said to be p-polarized because it is parallel to the
plane Light with the perpendicular polarization is said to be s-polarized from the German
senkrechtmdashperpendicular When unpolarized light strikes a surface at Brewsters angle the
reflected light is always s-polarized
The physical mechanism for this can be qualitatively understood from the manner in which
electric dipoles in the media respond to p-polarized light One can imagine that light incident on
the surface is absorbed and then reradiated by oscillating electric dipoles at the interface
between the two media The polarization of freely propagating light is always perpendicular to
the direction in which the light is travelling The dipoles that produce the transmitted (refracted)
light oscillate in the polarization direction of that light These same oscillating dipoles also
generate the reflected light However dipoles do not radiate any energy in the direction along
which they oscillate Consequently if the direction of the refracted light is perpendicular to the
direction in which the light is predicted to be specularly reflected the dipoles will not create any
reflected light Since by definition the s-polarization is parallel to the interface the
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials
MAHALAKSHMI ENGINEERING COLLEGE-TRICHY ELECTROMAGNETIC FIELDS
KSRINIVASAN APECE Page 23
corresponding oscillating dipoles will always be able to radiate in the specular-reflection
direction This is why there is no Brewsters angle for s-polarized light
With simple trigonometry this condition can be expressed as
where θ1 is the angle of incidence and θ2 is the angle of refraction
Using Snells law
we can calculate the incident angle θ1=θB at which no light is reflected
Rearranging we get
where n1 and n2 are the refractive indices of the two media This equation is known as
Brewsters law
Note that since all p-polarized light is refracted (ie transmitted) any light reflected from the
interface at this angle must be s-polarized A glass plate or a stack of plates placed at Brewsters
angle in a light beam can thus be used as a polarizer
For a glass medium (n2asymp15) in air (n1asymp1) Brewsters angle for visible light is approximately 56deg
to the normal while for an air-water interface (n2asymp133) its approximately 53deg Since the
refractive index for a given medium changes depending on the wavelength of light Brewsters
angle will also vary with wavelength
The phenomenon of light being polarized by reflection from a surface at a particular angle was
first observed by Etienne-Louis Malus in 1808 He attempted to relate the polarizing angle to the
refractive index of the material but was frustrated by the inconsistent quality of glasses available
at that time In 1815 Brewster experimented with higher-quality materials and showed that this
angle was a function of the refractive index defining Brewsters law
Although Brewsters angle is generally presented as a zero-reflection angle in textbooks from the
late 1950s onwards it truly is a polarizing angle The concept of a polarizing angle can be
extended to the concept of a Brewster wave number to cover planar interfaces between two
linear bianisotropic materials