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.
Spherical wavefront: Light diverging from a point source forms spherical wavefronts centered at the source, as all points equidistant from the source have the same phase.
Plane wavefront: When a point source is placed at the focus of a convex lens, the emerging wavefront is plane. This follows from the reversibility of light paths - the context shows a plane wave incident on a convex lens produces a spherical wave converging to focus.
Plane wavefront: A distant star is effectively a point source at infinity. The spherical wavefronts from such a distant source become essentially plane over the small region intercepted by Earth.
Explanation
The textbook context describes how a plane wave incident on a convex lens produces a spherical wave converging to the focus. By the principle of reversibility of light paths, when a point source is placed at the focus, the emerging wavefront becomes plane. For distant stars, the large distance makes the spherical wavefronts appear nearly plane over the small area intercepted by Earth.