Why theres a phase reversal when wave reflects from denser surface but no phase reversal when it reflects from rarer surface?

Hello Student,

If a wave reflects from an optically more dense medium than the one in which it was travelling (e.g when going from air to glass) then a phase change of π is produced, corresponding to an effective path difference of half a wavelength.
But no such things happens in case of a wave reflects from an optically less dense medium.

Thanks & Regards
Arun Kumar
Btech, IIT Delhi
Askiitians Faculty