The physical quantity which does not change when X-rays of wavelength \( 10^{-10}\,\mathrm{m} \), or visible light of wavelength 400 nm or radio waves of 500 m wavelength pass through glass is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → frequency
When a wave passes from one medium into another, its frequency is fixed by the source and does not change. The speed and the wavelength change with the refractive index \( n \) of the medium:
\( v = c/n \)
\( \lambda' = \lambda/n \)
Option (1) frequency — Correct. The number of wave crests reaching the boundary each second equals the number leaving it, so the frequency is the same in glass.
Option (2) wavelength — Incorrect. The wavelength in glass becomes \( \lambda/n \).
Option (3) speed — Incorrect. The speed in glass falls to \( c/n \).
Option (4) refractive index — Incorrect. The refractive index of glass depends on the wavelength (dispersion), so it is different for X-rays, visible light and radio waves.
NCERT (2026-27):
Refraction of a Plane Wave – “… The above equation implies that when a wave gets refracted into a denser medium (v1 > v2) the wavelength and the speed of propagation decrease but the frequency \( \nu \) (= v/λ) remains the same.”