(i) Action of the prism when a plane wavefront incident on it: In adjacent figure, consider a plane wave passing through a thin prism. Since the speed of light waves is less in glass, the lower portion of the incoming wavefront which travels through the greatest thickness of glass will get delayed resulting in a tilt in the emerging plane wavefront.
(ii) Action of the convex lens when a plane wavefront incident on it: In the adjacent figure, a plane wave incident on a thin convex lens; the central part of the incident plane wave traverses the thickest portion of the lens and is delayed the most. The emerging wavefront has a depression at the centre and therefore the wavefront becomes spherical (radius = f, focal length) and converges to the point focus F.
(iii) Action of the concave mirror when a plane wavefront incident on it: In adjacent figure, a plane wave is incident on a concave mirror and on reflection we have a spherical wave converging to the focus F.
