JEE Main · 2012 · 29 Apr 2012 · Offline · EM Waves

An electromagnetic wave in vacuum has the electric and magnetic fields

E

and

B

, which are always perpendicular to each other. The direction of polarization is given by

X

and that of wave propagation by

k

. Then :

(A) $X ∥ B$ and $k ∥ B \times E$
(B) $X ∥ E$ and $k ∥ E \times B$ ✓ Correct
(C) $X ∥ B$ and $k ∥ E \times B$
(D) $X ∥ E$ and $k ∥ B \times E$

Answer: (B)

Solution

By convention the direction of polarization of an electromagnetic wave is taken along its electric field, so X⃗ ∥ E⃗. The wave carries energy — and therefore travels — along the Poynting vector S⃗ = (1/μ₀) E⃗ × B⃗, so k⃗ ∥ E⃗ × B⃗. Only one option satisfies both: X⃗ ∥ E⃗ and k⃗ ∥ E⃗ × B⃗.

JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option

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