What is the shape of the wavefront in each of the following cases:

(a) Light diverging from a point source.

(b) Light emerges out of a convex lens when a point source is placed at its focus.

(c) The portion of the wavefront of the light from a distant star intercepted by the Earth.

(a) 

Hint: A wavefront is a surface of constant phase.
Step 1: Draw the shape of the wavefront.
When the light diverges from a point source, the shape of the wavefront is spherical. Following is the figure of the wavefront:

Wave Optics

(b)
Hint:
A wavefront is a surface of a constant phase.
Step 1: Draw the shape of the wavefront.
When the light is emerging from the convex lens, the shape of the wavefront is parallel odd. In this case, the point source is placed at its focus.

Wave front

(c)
Hint:
A wavefront is a surface of a constant phase.
Step 1: Identify the shape of the wavefront.
When the light is coming from a distant star that is intercepted by the earth, the shape of the wavefront is plane.