When light travels from one medium to another, then its

The correct answer is Option (2) → wavelength changes but frequency remains the same

The frequency of light is fixed by the source: the charges at the boundary oscillate at the frequency of the incoming wave and re-emit at that same frequency. The speed changes from medium to medium, so by \( v = f\lambda \) the wavelength changes in proportion to the speed.

    \( \displaystyle \lambda_{\mathrm{medium}} = \frac{v}{f} = \frac{\lambda_{\mathrm{air}}}{n} \)

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.”