What is the shape of the wavefront in each of the following cases ?
(a) light diverging from 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 earth.

Explanation

(a) The geometrical shape of the wave front would be diverging spherical wave front, as shown in Fig.
(b) When point source is placed at the focus of a convex lens, the rays emerging from the lens are parallel. Therefore, the wave front must be plane, as the shown in Fig.
( c) As the star (i.e. source of light) is very far off i.e. at infinity, the wave front intercepted by earth must be a plane wave front as shown in Fig.