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.

(N/A) The shape of the wavefront for light diverging from a point source is spherical.
$(b)$ The shape of the wavefront for light emerging from a convex lens when a point source is placed at its focus is plane (planar).
$(c)$ The portion of the wavefront of light from a distant star intercepted by the Earth is plane (planar) because the source is at an effectively infinite distance.