electromagnetic waves and the nature of light
DESCRIPTION
Reflection, refraction, dispersion, polarizationTRANSCRIPT
Electromagnetic Waves
Electromagnetic spectrum & Speed of light
The Electromagnetic Spectrum
• Light waves are electromagnetic waves• Light waves consist of periodically
changing electric and magnetic fields• Produced by accelerating charged
particles
The Electromagnetic Spectrum
• The complete range of frequencies and wavelengths of EM waves• Visible light is only a small portion of
the spectrum• Various kinds are classified according
to the methods by which they are generated or received
The Electromagnetic Spectrum
• Radio waves are produced by charges accelerating in a wire• Microwaves are used in radar
systems in air-traffic control, for transmitting long distance phone communications in outer space, and to cook food
The Electromagnetic Spectrum
• Infrared waves appear as heat when absorbed by objects• Visible light is produced by the
rearrangement of electrons in atoms and molecules–(λ)400-700nm VL human eye can
detect
The Electromagnetic Spectrum
• The ozone layer of the atmosphere filters practically all of the high frequency components of the UV radiation• The inner atmosphere readily
transmits the remaining lower frequency UV radiation
The Electromagnetic Spectrum
• X rays are used as diagnostic tools by Physicians, but can destroy living tissues and organisms• Gamma rays are emitted by
radioactive nuclei
Speed of Light
• c represents the speed of lightc = 3.00x108
• The speed of light in a material is always less than c• v=fλ applies to light waves
The nature of light; Reflection and Refraction; Total Internal Reflection; Dispersion; Polarization;
Scattering of Light
Nature and Propagation of Light
Light: Particle or Wave?
• Light as a wave–Rectilinear propagation –Can be reflected–Can be refracted
• Photoelectric effect–Particle behavior– Increased intensity increases number of
electrons emitted
The facts of Light
• The ability of things to light–Bioluminescence
• Anything that can give off its own light–Luminous object
• An object that can be seen because of a luminous object– Illuminated object
The facts of Light
• The rate at which light is emitted from a source –Luminous flux (P) measured in
lumen (lm)
• Illumination on a surface–Illuminance, (E) measured in lm/m2
or lux (lx)
1m
2m
3m
1
1/9
1/4
S
REFLECTION & REFRACTIONBehaviours of light
Reflection
Wave changes direction upon hitting a reflecting surface
0ri
R
N
I
R= reflecting rayr= angle of reflectionI= incident rayi= angle of incidenceN= normal
Law of Reflection
θi= θr
States that waves are reflected from the boundary of the medium at the same
angle at which they strike it
RefractionWave changes direction upon entering a medium of different density
Absolute index of Refraction
• n is the ratio of the speed of light in a vacuum, c, to the speed of light in a material medium, v• n=c/v
n2= v1= λ1
n1 v2 λ2
Snell’s law/ law of refraction
The ratio of the sines of the angles, where both angles are measured from the normal to the surface, is equal to the inverse ratio of the two indexes of refraction
sinθ2= n1sinθ1 n2
Index of refraction and the wave aspects of light
λ=λ0/n
f
Frequency of the wave does not change when passing from
one material to another
λ
The wavelength of the wave is different in general in different
materials
As the λ gets shorter, the v decreases
As the λ gets longer, the v increases
TOTAL INTERNAL REFLECTION
Total Internal Reflection
When does total internal reflection occur? What are the conditions?
Total Internal Reflection
When light emerges into a
less dense medium, rays
bend away from the normal
As incident rays become
more oblique, angle of
refraction approaches a maximum of
90°
Total Internal Reflection
The angle of incidence for
which the refracted ray
emerges tangent to the surface is called critical
angle
The incident angle when θr is
90° (θcrit)
Total Internal Reflection
If θr is beyond 90°, then no refracted ray would exist. TIR
occurs
sin θ crit = nb/na
Critical angle for TIR
Total Internal Reflection
TIR occurs only when a ray is incident on the interface with a
second material whose n is smaller than that of the material
in which the ray is travelling
If θincidence ≥ θcritical
DISPERSION
Dispersion
Speed of light in a material in different
substances is different for different wavelengths The index of
refraction of a material
depends on wavelength
The dependence of wave speed and index of
refraction on wavelength is called dispersion
In most material, n inc with decreasing wavelength and inc f
Light of longer wavelength has greater v than light of shorter wavelength
In the visible light in a prism, violet light is deviated most and red light is deviated least
Light spreads out and it is said to be dispersed into a spectrum
Dispersion
DispersionThe amount of
dispersion depends on the
difference between the refractive
indexes for violet light and
for red light
POLARIZATION
Polarization
• It is a characteristic of all transverse waves
• An EM wave is produced by fluctuating electric and magnetic fields which are perpendicular to each other and to the direction of propagation
• Electromagnetic wave is a transverse wave
Polarization
Waves on a confined plane is
said to be plane
polarized
The alignment of
transverse electric vectors
Such waves of aligned
vibrations are said to be polarized
Polarization
For a string that’s on equilibrium along the x-axis, the displacements may be along the y-
direction or along the z-direction
XZ
Y
XZ
Y
Polarization
• When a wave has only y-displacement, then it’s linearly polarized in the y-direction• When a wave has only z-
displacement, then it’s linearly polarized in the z-direction
Polarizing filter or polarizer
It only permits mechanical waves with a certain polarization direction to pass
XZ
Y
Unpolarized light
Polarization
Polarization by Reflection
Reflected light may be partially polarized
Polarization by Reflection
The refracted
light is partially polarized
The Polarizing Angle
θp θp
θb
At the polarizing angle, the reflected light is 100% polarized and the refracted light is partially polarized
The Polarizing Angle• When θi is equal to θp, the
reflected and refracted rays are perpendicular to each other• θb becomes the complement of θp
• So θb= 90°- θp
• Tan θp= nb/na (Brewster’s Law)
Sunglasses use Polaroid filters that have their molecules aligned horizontally (parallel to the ground), with a polarizing axis along the vertical
Polarizing filter or polarizer
Polaroid
Dichroism, a selective
absorption
Transmits light parallel to a
Polarizing axis
THE SCIENCE OF COLORROYGBIV
Basic Color Principles
Color dimensions vary
Color dimensions vary
Additive Primary Colors
• These three colors can approximate, through selective mixture, all of the other colors.
BLUE
RED GREENW
Secondary Colors
• Each is placed between the two primaries that are mixed to produce it
M C
Y
Primary & Secondary Colors
Red Blue magenta
Primary & Secondary Colors
Red Green yellow
Primary & Secondary Colors
Green Blue Cyan
Complementary
features colors
directly opposite
on the color wheel
White
Red
yellow
Green
cyan
Blue
magenta
Complementary
•When two colors are added together to produce white•Magenta + green = white• Cyan + red = white• Yellow + blue = white
Mixing Colored Pigments
• Red + Green + Blue = muddy dark brown• Rule for adding colored light
does not apply• Pigments are tiny particles that
absorb specific colors
Mixing Colored Pigments
• Magenta, Cyan, and Yellow are subtractive primary colors
M C
Y
SCATTERING OF LIGHTColors and the colors of the atmosphere
Scattering of Light
• The absorption and re-radiation of light in a variety of directions is called scattering
• Scattering of light depends on λ
Scattering of Light
The smaller particles will be less of an obstruction to long λ than to short ones
As the beam of light passes through the atmosphere, its intensity decreases
The intensity of light scattered from air molecules inc in proportion to the fourth power of the frequency
Scattering of Light: Why the sky is blue
The intensity of light scattered from air molecules is inversely proportional to the fourth power of the λ
Intensity ratio for the 2 ends of the spectrum: (700nm/400nm)4= 9.4
Scattering of Light: Why the sky is white
Clouds contain high concentration of water droplets
Light of all wavelengths are scattered out of the cloud, so the cloud looks white
Scattering of Light: Why sunsets are red?
Light has to travel longer distance
A substantial fraction of the blue light is removed by scattering: W-B= Y or R