Question

Why is there a change in phase upon reflection of light in a mirror?

Answer 1

This is an excellent question that might have been better posed in the physics section.........

Explanation:

As an ignorant chemist, I will do my best to muddy the waters.

The very best mirrors are still made by a classic inorganic reaction: the `"Tollens' reagent"`, i.e. reduction of ammoniacal silver nitrate..........

`2 [Ag(NH_3)_2]^+ + RC(=O)H + H_2O → 2Ag(s)darr + 4NH_3 + RC(=O)OH + 2 H^+`

The metal surface (which is not a monolayer in the best circumstances) reflects incident light with a `180""^@` change of phase......The monolayer is MUCH thicker than the gold film of Rutherford's experiment, which was only a few atoms thick at most.

Here is a link to a Scientific American article, which does not wholly satisfy me. There are also a series of answers here which I think are a bit more accessible.