Question

Why due to reflection phase change by 180?

Answer 1

Light waves change phase by 180° when they reflect from the surface of a medium with higher refractive index than that of the medium in which they are travelling. ... The phase changes that take place upon reflection play an important part in thin film interference.

Answer 2

A light wave travelling in air that is reflected by a glass barrier will undergo a 180° phase change, while light travelling in glass will not undergo a phase change if it is reflected by a boundary with air. ... The phase changes that take place upon reflection play an important part in thin film interference.