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NCERT Class XII Chapter
Electromagnetic Waves
Questions With Solutions

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Question : 11
Total: 15
Suppose that the electric field part of an electromagnetic wave in vacuum is E = [(3.1 N/C)] cos[(1.8 rad/m) y + (5.4 × 108 rad/s)t)]
^
i
.
(a) What is the direction of propagation?
(b) What is the wavelength λ?
(c) What is the frequency u?
(d) What is the amplitude of the magnetic field part of the wave?
(e) Write an expression for the magnetic field part of the wave.
Solution:  
(a) Electromagnetic wave is propagating in –y direction.
(b) Propagation constant, k =
2π
λ
 = 1.8
λ =
2π
1.8
 =
π
0.9
 m = 3.49 m
(c) Angular frequency, ω = 2πu= 5.4 × 108
v =
5.4
2×3.14
 × 108 = 86 × 106 Hz = 86 M Hz
(d) Amplitude of magnetic field part,
E0
B0
 = c
So, B0 =
E0
c
 =
3.1
3×108
 = 1.03 × 10−8 T = 10.3 × 10−9 T = 10.3 nT
(e) Expression for magnetic field part of wave
→
Bz
 = B0 cos (ky + ωt)
^
k
 = 1.03 × 10–8 cos (1.8y + 5.4 × 108 t)
^
k
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