What is the ratio of amplitude of magnetic field to the amplitude of electric field for an electromagnetic wave propagating in vacuum?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Reciprocal of speed of light in vacuum
In an electromagnetic wave in vacuum the amplitudes of the electric and magnetic fields are related through the speed of light, \( E_0 = cB_0 \). Hence the ratio asked for is
\( \displaystyle \frac{B_0}{E_0} = \frac{1}{c} \)
Why the other options are incorrect
Option (1) The ratio of magnetic permeability to the electric susceptibility of vacuum — Incorrect. The ratio \( B_0/E_0 \) has the unit \( \mathrm{s\,m^{-1}} \) (tesla per volt per metre), while this quantity does not.
Option (2) Unity — Incorrect. \( B_0 \) and \( E_0 \) are different quantities with different units; in SI units they are not equal in number.
Option (3) Speed of light in vacuum — Incorrect. This is the inverse ratio, \( E_0/B_0 = c \).
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
\( \displaystyle B_0 = (E_0/c) \qquad (8.10) \)”