The speed of light in water is less than the speed of light in air,confirms the assumption of which theory of light?

What is the shape of the wavefront in each of the following cases:
$(a)$ Light diverging from a point source.
$(b)$ Light emerging out of a convex lens when a point source is placed at its focus.
$(c)$ The portion of the wavefront of light from a distant star intercepted by the Earth.

$A$ wavefront is a surface

Huygens' theory of secondary waves can be used to find:

Is Huygens' principle valid for longitudinal sound waves?

For light diverging from a finite point source,