Let \( v_g \), \( v_x \) and \( v_r \) be the speeds of gamma rays, X-rays and radio waves in vacuum. Which of the following is correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( v_g = v_x = v_r \)
Gamma rays, X-rays and radio waves are all electromagnetic waves. They differ only in wavelength and frequency. In vacuum every one of them travels with the same speed \( c=3\times10^{8}\ \mathrm{m\,s^{-1}} \).
Option (1) \( v_g \lt v_x \lt v_r \) — Incorrect. Radio waves have a much longer wavelength, but a longer wavelength does not give a higher speed in vacuum.
Option (2) \( v_g = v_x = v_r \) — Correct. All three travel through vacuum with the speed \( c \), so the three speeds are equal.
Option (3) \( v_g \lt v_x \gt v_r \) — Incorrect. X-rays are not faster than gamma rays or radio waves. No part of the spectrum moves faster than another in vacuum.
Option (4) \( v_g \gt v_x \gt v_r \) — Incorrect. Gamma rays have the highest frequency. Since \( \nu\lambda=c \), a higher frequency only means a shorter wavelength, not a higher speed.
NCERT (2026-27):
Points to Ponder – “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.”