Clearly, 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 wavefront as shown in the figure In Fig 10 7(b) we
consider 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