Light's Refractive Index & Phase Change: Explained

[manimaran1605]

Consider a plane beam of light is falling normally on a material with thickness (d) and refractive index (n), the light partially transmits and partially reflects the light, so the light transmitting the light travels a distance (2nd) coming out of the material have a phase change of ∏ always, How? will it be different for different thickness?
If no why the phase change can't be ∏/2 or 2∏ or something else?

 

[Simon Bridge]

There is usually a phase change associated with reflection - either zero or pi, depending on the refractive index either side of the surface.

The phase shift for light traversing a medium depends on the thickness of the medium and the refractive index.