What happens when light is refracted into a medium?

The correct answer is Option (3) → The wavelength changes but the frequency remains unchanged

The frequency of light is fixed by the source and does not change on refraction, because each point of the boundary is set into oscillation by the incident wave and re-emits at the same frequency. The speed changes to \( v = c/n \), so the wavelength changes in the same ratio.

    \( \displaystyle \lambda = \frac{v}{\nu} = \frac{\lambda_0}{n} \)

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.”