When a ray of light enters a glass slab from air:

The correct answer is Option (1) → its wavelength decreases

The frequency of light is fixed by the source and does not change at the boundary. The speed in a medium of refractive index n is \( v = c/n \), and glass has \( n \gt 1 \) (about 1.5), so the speed falls. Since \( v = f\lambda \) and f is constant, the wavelength decreases in the same ratio:

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

Note: A similar question was also asked in CUET (UG) 2026 on 26-05-2026 (Slot 1).

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