Identify the correct statements among the following.
A. All electromagnetic waves, irrespective of their frequencies, travel at the same speed through vacuum.
B. A capacitor is fully charged by a d.c. source. When it is fully charged, the displacement current is zero.
C. Ultraviolet radiation can be used for viewing objects through haze and fog.
D. A moving charge in a circular orbit cannot produce electromagnetic waves.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A and B only
(A) All electromagnetic waves, irrespective of their frequencies, travel at the same speed through vacuum. — Correct. In vacuum every electromagnetic wave travels at \( c = 1/\sqrt{\mu_0\varepsilon_0} = 3\times10^{8}\,\mathrm{m\,s^{-1}} \).
(B) A capacitor is fully charged by a d.c. source. When it is fully charged, the displacement current is zero. — Correct. The charge, and so the field between the plates, no longer changes, so \( i_d = \varepsilon_0\,d\phi_E/dt = 0 \).
(C) Ultraviolet radiation can be used for viewing objects through haze and fog. — Incorrect. Infrared radiation is used for this: its longer wavelength is scattered much less by haze and fog.
(D) A moving charge in a circular orbit cannot produce electromagnetic waves. — Incorrect. A charge in a circular orbit is accelerated (centripetal acceleration), and every accelerated charge radiates electromagnetic waves.
NCERT (2026-27):
Points to Ponder – “1. The basic difference between various types of electromagnetic waves lies in their wavelengths or frequencies since all of them travel through vacuum with the same speed.”
Points to Ponder – “… 2. Accelerated charged particles radiate electromagnetic waves.”