A plane electromagnetic wave of frequency 25 MHz travels in vacuum. At a certain point in its path, the electric field of the wave has its maximum value at the instant \( t=0 \). The earliest instant after \( t=0 \) at which the magnetic field at the same point becomes zero is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 10 ns
The electric and magnetic fields of a plane wave vary as the same sine function of \( (kz-\omega t) \), so they are in phase. At a fixed point, B is maximum when E is maximum. B becomes zero when E becomes zero, a quarter of a period after a maximum.
The period of the wave is:
\( \begin{aligned} T &= \frac{1}{\nu} \\ &= \frac{1}{25\times10^{6}\,\mathrm{Hz}} \\ &= 4\times10^{-8}\,\mathrm{s} \end{aligned} \)
The fields fall from their maximum to zero in a time:
\( \begin{aligned} t &= \frac{T}{4} \\ &= \frac{4\times10^{-8}\,\mathrm{s}}{4} \\ &= 10^{-8}\,\mathrm{s} \\ &= 10\,\mathrm{ns} \end{aligned} \)
NCERT (2026-27):
Nature of Electromagnetic Waves – “… The electric and magnetic fields Ex and By are perpendicular to each other, and to the direction z of propagation. We can write Ex and By as follows:
\( E_x=E_0\sin(kz-\omega t) \)\( \qquad [8.7(a)] \)
\( B_y=B_0\sin(kz-\omega t) \)\( \qquad [8.7(b)] \)”