Huygens' wave theory allows us to know

Assertion $A$: For light diverging from a point source,the intensity at the wavefront does not depend on the distance.
Reason $R$: In a diverging beam of light from a point source,a spherical wavefront is observed.

Carefully analyze the given diagrams and choose the correct option regarding the refraction of a plane wavefront by a thin convex lens:

The idea of secondary wavelets for the propagation of a wave was first given by

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' principle is applicable to