The electric field of a plane polarized electromagnetic wave in free space at time t=0 is given by the expression E(x, y)=10 jcos(6x+8z). The…
JEE Main 2019 — Physics Electromagnetism
2019mcqhard
The electric field of a plane polarized electromagnetic wave in free space at time t=0 is given by the expression E(x,y)=10j^cos(6x+8z). The magnetic field B(x,z,t) is given by (c is the velocity of light.)
Official previous-year question
Held on 10 Jan 2019 · Verified 6 Jul 2026.
Options
A
c1(6k^−8i^)cos(6x+8z+10ct)
B
c1(6k^+8i^)cos(6x+8z−10ct)
C
c1(6k^+8i^)cos(6x−8z+10ct)
D
c1(6k^−8i^)cos(6x+8z−10ct)
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Solution
E=E0cos(ωt−K⋅r)
∴ At any time t,
K=6i^+8k^
B0=cE0=c10
λ=k2π=102π=5π
Also, E×B=c
∴ω=2πf=λ2πc
∣i^0Bxj^1Byk^0Bz∣=106i^+8k^
ω=(5π)2π×c=10c
∴i^[Bz−0]+j^(0)+k^(−Bx)=0.6i^−0.8k^
Comparing the coefficients on both sides,
∴Bz=0.6;Bx=−0.8
∣B0∣=c10(−0.8i^+0.6k^)=c−8i^+6k^
So the magnetic field,B=c1(−8i^+6k^)cos(6x+8z−10ct)tesla
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