Two plane electromagnetic waves travel in vacuum. The peak electric field of the first wave is twice that of the second, and the frequency of the first wave is one-third of the frequency of the second. The ratio of the peak magnetic field of the first wave to that of the second wave is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2 : 1
In vacuum, the peak fields of an electromagnetic wave follow \( B_0=E_0/c \). The speed \( c \) is the same for every frequency. So the peak magnetic field depends only on the peak electric field, and the frequencies play no part.
For the two waves:
\( \begin{aligned} \frac{B_{01}}{B_{02}} &= \frac{E_{01}/c}{E_{02}/c} \\ &= \frac{E_{01}}{E_{02}} \\ &= \frac{2}{1} \end{aligned} \)
Therefore, the peak magnetic fields are in the ratio 2 : 1.
NCERT (2026-27):
Nature of Electromagnetic Waves – “… It is also seen from Maxwell’s equations that the magnitude of the electric and the magnetic fields in an electromagnetic wave are related as
\( B_0=(E_0/c) \)\( \qquad (8.10) \)”