Which of the following has zero average value in a plane electromagne…

The correct answer is Option (4) → (A) and (B) only

In a plane electromagnetic wave the fields vary sinusoidally, \( E = E_0\sin(kx - \omega t) \) and \( B = B_0\sin(kx - \omega t) \). A sine is positive for half a cycle and negative for the other half, while the energy densities depend on the squares of the fields and are never negative.

(A) Electric field — Correct. The average of \( \sin(kx - \omega t) \) over a cycle is zero.

(B) Magnetic field — Correct. It varies in the same way as \( E \), so its average is also zero.

(C) Electric energy — Incorrect. The energy density \( \frac{1}{2}\varepsilon_0 E^{2} \) averages to \( \frac{1}{4}\varepsilon_0 E_0^{2} \), not zero.

(D) Magnetic energy — Incorrect. The energy density \( B^{2}/2\mu_0 \) averages to \( B_0^{2}/4\mu_0 \), not zero.

Note: A similar question was also asked in CUET (UG) 2026 on 31-05-2026 (Slot 2).