Solution Explanation

Among the four listed phenomena:

Reflection of light

Wave theory (Huygens’s principle, for example) adequately explains reflection.

Refraction of light

Wave theory also explains refraction via the change of wave speed in different media.

Diffraction of light

Wave theory (again, Huygens–Fresnel principle) explains the bending of waves around obstacles.

Compton effect

This involves inelastic scattering of photons by electrons (where the photon loses energy and is scattered with a longer wavelength).

Wave theory alone cannot explain the discrete energy exchange and wavelength shift; it requires the particle (photon) concept from quantum theory.

Hence, the phenomenon not explained by the (classical) wave theory is the Compton effect.

Answer: (D) Compton effect.