When light is refracted from one medium into another,

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

The frequency of light is fixed by its source. When light goes into another medium, its speed changes. Since \( v=\nu\lambda \), the wavelength changes along with the speed.

Option (1) both the wavelength and the frequency increase — Incorrect. The frequency does not change on refraction. Also, in a denser medium the wavelength becomes smaller.

Option (2) the wavelength increases and the frequency decreases — Incorrect. The frequency stays the same. The wavelength becomes larger only when light goes into a rarer medium.

Option (3) the wavelength changes but the frequency remains the same — Correct. The speed and the wavelength change together, in the same ratio. The frequency \( \nu=v/\lambda \) keeps its value.

Option (4) nothing changes — Incorrect. The speed of light is different in the two media. So the wavelength must change too.

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.”