Which of the following change is observed when light travels from air to glass?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → wavelength and speed of light only
The frequency of light is fixed by the source and does not change when light passes from one medium to another. In glass of refractive index \( n \), the speed falls to \( v = c/n \), and since \( v = f\lambda \) with \( f \) unchanged, the wavelength falls in the same ratio.
\( \displaystyle \lambda_{\mathrm{glass}} = \frac{v}{f} = \frac{\lambda_{\mathrm{air}}}{n} \)
Option (1) speed of light only — Incorrect. The speed does change, but the wavelength changes along with it, so "only" is wrong.
Option (2) frequency of light only — Incorrect. Frequency is the one quantity that does not change on refraction.
Option (3) wavelength of light only — Incorrect. The wavelength changes, but so does the speed, so "only" is wrong.
Option (4) wavelength and speed of light only — Correct. Both the speed and the wavelength fall by the factor \( n \), while the frequency stays the same.
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 ν (= v/λ) remains the same.”