The wave front is a surface in which

State and explain Huygens' principle.

Explain the refraction of a plane wave from a denser to a rarer medium using Huygens' principle.

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)

Huygens' concepts of secondary wavelets:

For light diverging from a point source,