physics electromagnetic waves

7/28/2019 Physics Electromagnetic Waves

1/6


2/6

9 -ELECTROMAGNETIC WAVESto MN as shown In the figure. Similarly,the electric field Intensity at D Is alsoparallel to MN but Is of the smallermagnitude. Thus there Is a gradualdecrease In the Intensity of the electricfield at a given Instant as we move awayfrom MN.As the spark Is produced In the sparkgap, electrons flow from the sphere Q1 toQ2 reducing negative charge on Q1 andpositive charge on Q2 . With one halfcycle of time elapsing, the charge on Q1becomes positive and that on Q2negative. Now the electric fields at C andD are In opposite directions.

The Process of EmissionThe Hertzlan dipole Isshown In the figure.

p he Inthe paperand linestheare( a ) and ( b )the state of thedipole and thecorresponding electricand magnetic field linesat times t =0 andt =T I 8 respectively.

At time t = T /4 , thedipole moment becomeszero. In this case, theelectric and the magnetic

In vertical

( a ) t = 0

E

3T(d)=u

M

s

+

T(b ) t = 8E

4T( e ) t=-8

B( c I t =

Page 2

2T8

B


3/6


4/6


5/6

( lv )( y )

(v i )

( vi i )

9 ELECTROMAGNETIC WAVES Page 5Electromagnetic waves are transverse waves.Electromagnetic waves possess energy.

Electromagnetic waves exert pressure on a surface and Impart linear momentum to Iwhen they are Incident on it.If U Is the energy of electromagnetic waves Incident on a surfacesecond normal to I t and is completely absorbed, then pressure exertp = which Is also the momentum of electromagneUc r a d l a t l o ~Electromagnetic field prevails In the region where the e l e c t r&a .Y . a ves propagateThe electromagnetic energy per unit volume In the region ( ~ d e n s i t y )

1 2 e2P = PE + Pa = -So E + -- 0 +2 2 llooThis formula Is based on formulae for energyG r and a solenoid where thefields are stationary. In electromagnetic wave flel oscillate as per sine or cosinefunction. Hence replacing them by t h ~ e l r s

1 2 erms2 . .VP = 2So Erms + 2 1'0ErmsPutting B rm = ,

P = ~ S o . : \ '!J;. Soc2 = 1 s E 2 1 s E 22 2 c2 2 0 rms + 2 0 rmsD p = stll( vii i ) " T ~ e lty ~ radiation ( l ) Is defined as the radiant energy passing through uniarea to the direction of propagation In one second.

= nergy I time = PowerArea Area - - - - - - - ~' 'I '1

hown In the figure, the radiant _ ) ~ \ . / \ :,-='1-if:\7-"\ r=(:ft.\ :1\energy passing through unit area f\ - - . . ~ : , ~ /\'-'1\ ~ . } , - \ ~ . ~In one second is confined to a .J"' ( \ ' .,_,.,.....'-' .. ._,_ ..V K - ' I " / " " ' . ~volume of length equal to c. I f \.../ ~ : / J V\._ / ' J : ,V \ .-tp is the energy density, then the ~ - - - - - - - ~energy In th e above volume = p +----- c - - ~c .

0 I = p c = So c Erms2( lx ) In the region fa r away from the source, electric and magnetic fields oscillate In phase

and are called radiated components of electromagnetic radiation.


6/6

9 - ELECTROMAGNETIC WAVES Page 69.4 Electromagnetic SpectrumThe electromagnetic: waves have wavelengths ranging from 10"15 m to 108 m. Human eyes aresensitive to visible l ight having wavelengths ranging from 4000 A to 8000 A. Thec:lassific:atlon of electromagnetic: waves Is referred to as the electromagnetic Theelectromagnetic: waves in increasing order of wavelengths and decreasing( i ) y-rays, ( i i ) X-rays, ( IIi ) ultraviolet rays, ( lv ) visible light, ( v) infrared1 vi ) m ic:rowaves, ( vi i ) short radio waves and ( viii ) long radiowavelengths less than 1 A whereas radio waves have wavelengths mcorlllll'lllllllno sharp boundaries dividing the various sections of the e i E t c : t o o n 1 a ~ 1 n e t t i 4 J9.5 Electromagnetic radiation and Earth's atmosphere

have

Processes like reflection, refraction, polarization, dispersionelectromagnetic rays coming from the Sun pass throughand reach the surface of the Earth. The Earth'sabout 15 km), Stratosphere 11!; to 50 km), Mesosphere110 km ). The important points to be noted about the

take place whenin Earth's atmosphereof Troposphere (uptoand Thermosphere (80 toatmosphere are:

11 ) The density of the atmosphere decreases ~ ~ h i ! l h , ~ r . There Is no sharp boundarybetween the different layers. . ,V

(2) positive ions.13 ) The layers other than the ionos re e electrically neutral.

physics electromagnetic waves

Documents