When light propagates through a given homogeneous medium,the velocities of

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.

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

When wavefronts pass from a denser medium to a rarer medium, the width of the wavefront

Given below are two statements:
Statement-$I$: $A$ plane wave after passing through a prism remains a plane wave, but passing through a small pinhole may become a spherical wave.
Statement-$II$: The curvature of a spherical wave emerging from a slit will increase for increasing slit width.
In the light of the above statements, choose the correct answer from the options given below.

Huygens' concepts of secondary wavelets: