When light enters from air to glass, its

The correct answer is Option (3) → wavelength decreases

The frequency is fixed by the source and does not change on refraction. The speed falls from c to \( v = c/n \), so the wavelength falls in the same ratio.

    \( \displaystyle \lambda_{\mathrm{glass}} = \frac{v}{\nu} = \frac{\lambda_{\mathrm{air}}}{n} \)

With \( n \approx 1.5 \), the wavelength in glass is about two-thirds of that in air.

Why the other options are incorrect

• Option (1) frequency increases — Incorrect. Frequency does not change when light crosses a boundary.

• Option (2) frequency decreases — Incorrect. Frequency does not change when light crosses a boundary.

• Option (4) wavelength increases — Incorrect. Glass is optically denser, so speed and wavelength both decrease.

Note: A similar question was also asked in CUET (UG) 2023 on 09-06-2023 (Slot 3).

NCERT (2026-27):
Refraction of a Plane Wave – “…

    \( \displaystyle \frac{v_1}{\lambda_1} = \frac{v_2}{\lambda_2} \qquad (10.7) \)

The above equation implies that when a wave gets refracted into a denser medium \( (v_1 \gt v_2) \) the wavelength and the speed of propagation decrease but the frequency \( \nu \) \( (= v/\lambda) \) remains the same.”