$A$ wavefront is the locus of all points where the particles of the medium vibrate with the same:

What does Huygens' principle explain regarding secondary wavelets?

The following figure represents a wave front $AB$ which passes from air to another transparent medium and produces a new wave front $CD$ after refraction. The refractive index of the medium is $:$ ($PQ$ is the boundary between air and the medium)

By Huygen's wave theory of light,we cannot explain the phenomenon of

Explain the use of wavefront to understand wave propagation.

Derive the laws of refraction using the concept of Huygens' principle of wavefronts.