Answer:
When a wave is reflected into rarer medium from the surface of a denser medium, it undergoes a phase change of π radian or 180°
Explanation:
The topic here is optics.
Let's say light of any other wave per say is traveling in air and it's reflection is caused by glass, the light wave will alters its phases by 180° or π rad but if it's travelling from glass and it's reflected by air, it'll not change its phases.
This means that optical boundaries are normally stated in pairs.
Glass-air means from glass to air
Air-glass means from air to glass
