For electromagnetic waves propagating in vacuum, choose the correct statements.
A. frequency increases from radio waves to gamma rays.
B. wavelength increases from radio waves to gamma rays.
C. speed increases from radio waves to gamma rays.
D. speed remains constant
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and D only
All electromagnetic waves travel in vacuum with the same speed \( c = 3\times10^{8}\,\mathrm{m\,s^{-1}} \), and \( c = \nu\lambda \). Going from radio waves through microwaves, infrared, visible, ultraviolet and X-rays to gamma rays, the frequency increases, so the wavelength must decrease.
(A) frequency increases from radio waves to gamma rays. — Correct. Gamma rays have the highest frequency in the spectrum and radio waves the lowest.
(B) wavelength increases from radio waves to gamma rays. — Incorrect. Since \( \lambda = c/\nu \) with c fixed, the wavelength decreases as the frequency increases.
(C) speed increases from radio waves to gamma rays. — Incorrect. In vacuum every electromagnetic wave travels at c, whatever its frequency.
(D) speed remains constant — Correct. The speed in vacuum is \( c = 1/\sqrt{\mu_0\varepsilon_0} \) for all electromagnetic waves.
NCERT (2026-27):
Electromagnetic Spectrum – “… We briefly describe these different types of electromagnetic waves, in order of decreasing wavelengths.”
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.”