Question

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.

Answer 1

a)

Since, the locus of all points equidistant from the point source is a sphere therefore, the shape of the wavefront in case of light diverging from a point source will be spherical.

The shape of the wavefront is shown in figure below.

b)

When a point source is placed at the focus of a convex lens, the rays of light coming out of the lens are parallel. Hence, the geometrical shape of wavefront will be a plane wave front.

The shape of the wavefront is shown in figure below.

c)

Star is considered as a point source of light. Since, it is very far away from earth, so the spherical wavefronts of star reaching to the earth are very large spheres. As we see only a small area of this large sphere therefore, it looks like a plane wavefront.