When a light wave in a rarer medium is reflected from the surface of an optically denser medium, it suffers a phase change of (in radian)

According to electromagnetic theory When a light beam falls on a boundary of two media, then one part of it will be reflected and other part will be refracted. If refractive index of a medium, where the electromagnetic wave is refracted, is greater than the medium incident, then phase of reflected ray will be reverse with respect to the incident beam. As a result of which the wave suffers a phase change of π.