Consider the following statements about electromagnetic waves and capacitor charging:
A. All electromagnetic waves, irrespective of their frequencies, travel at the same speed through vacuum.
B. The displacement current is zero when the capacitor is fully charged by a d.c. source.
C. Microwaves are produced by the accelerated motion of charges in conducting wires.
D. Ultraviolet radiation can be used for viewing objects through haze and fog.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A and B only
(A) All electromagnetic waves, irrespective of their frequencies, travel at the same speed through vacuum. — Correct. The speed in vacuum is \( c = 1/\sqrt{\mu_0\varepsilon_0} \), which contains no frequency.
(B) The displacement current is zero when the capacitor is fully charged by a d.c. source. — Correct. The displacement current is \( i_d = \varepsilon_0\,d\Phi_E/dt \). Once the capacitor is fully charged, the field between the plates no longer changes, so \( i_d = 0 \) (and the conduction current in the wires is zero too).
(C) Microwaves are produced by the accelerated motion of charges in conducting wires. — Incorrect. Accelerated charges in conducting wires (antennas) produce radio waves. Microwaves are produced by special vacuum tubes such as klystrons and magnetrons, and by Gunn diodes.
(D) Ultraviolet radiation can be used for viewing objects through haze and fog. — Incorrect. Short-wavelength ultraviolet is strongly scattered and absorbed by haze and fog. It is infrared, with its longer wavelength and much weaker scattering, that is used for photography through haze and fog.
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.”
Displacement Current – “… The current, given by Eq. (8.4), is a new term, and is due to changing electric field (or electric displacement, an old term still used sometimes). It is, therefore, called displacement current or Maxwell’s displacement current.”
Radio Waves – “Radio waves are produced by the accelerated motion of charges in conducting wires.”
Microwaves – “Microwaves (short-wavelength radio waves), with frequencies in the gigahertz (GHz) range, are produced by special vacuum tubes (called klystrons, magnetrons and Gunn diodes).”