An electromagnetic wave moves in vacuum. Its speed is (c), choose the correct options about the characteristics of electromagnetic waves
(A) \( c = \displaystyle\frac{1}{\sqrt{\mu_0 \epsilon_0}} \)
(B) electromagnetic waves are transverse in nature.
(C) both the electric field and the magnetic field vibrate perpendicular to the direction of propagation of waves.
(D) electromagnetic waves are longitudinal in nature.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (C) only
In an electromagnetic wave the electric and magnetic fields oscillate perpendicular to each other and to the direction of propagation, and the wave travels in vacuum at the speed fixed by \( \mu_0 \) and \( \epsilon_0 \).
(A) \( c = \displaystyle\frac{1}{\sqrt{\mu_0 \epsilon_0}} \) — Correct. This follows from Maxwell's equations and gives the speed of light:
\( \displaystyle c = \frac{1}{\sqrt{4\pi\times10^{-7} \times 8.85\times10^{-12}}} \approx 3\times10^{8}\,\mathrm{m\,s^{-1}} \)
(B) electromagnetic waves are transverse in nature. — Correct. The oscillating fields are perpendicular to the direction of travel.
(C) both the electric field and the magnetic field vibrate perpendicular to the direction of propagation of waves. — Correct. For a wave along \( z \), \( \vec{E} \) may lie along \( x \) and \( \vec{B} \) along \( y \); both are perpendicular to the propagation.
(D) electromagnetic waves are longitudinal in nature. — Incorrect. No field component oscillates along the direction of travel, so the waves are transverse, not longitudinal.
NCERT (2026-27):
Nature of Electromagnetic Waves – “It can be shown from Maxwell’s equations that electric and magnetic fields in an electromagnetic wave are perpendicular to each other, and to the direction of propagation.”
Nature of Electromagnetic Waves – “… Using Eqs. [8.7(a) and (b)] for Ex and By and Maxwell’s equations, one finds that \( \omega = ck \), where,
\( \displaystyle c = 1/\sqrt{\mu_0 \varepsilon_0} \qquad [8.9(a)] \)”
Introduction – “… Finally in Section 10.7 we will discuss the phenomenon of polarisation which is based on the fact that the light waves are transverse electromagnetic waves.”