I got the idea to post this thread from "Silver mirror? Illusion?
What actually happens when something reflects photons, when the photons hit, what happens?
In that thread someone said something abt metals reflecting…but what abt non-metals?
Well?
Aaargh! I had a really nice answer to this all written down. It was a marvelous piece of writing, with nice coloured diagrams in ASCII codes, but I closed the window before submitting!
I’m not going to write the entire post again, but suffice it to say that it’s note very complicated, once you accept the following:
[ul]
[li]Regard the light as a wave[/li][li]Assume a perfectly conducting wall[/li][li]Only look at the electrical field[/li][li]Use Maxwells laws at the interface[/li][/ul]
You get after not very much calculus that there will be a reflected electromagnetic wave, that is phase-shifted 180[sup]o[/sup].
Analogy:
You must have played with waves in high-school physics. There is a very famous experiment, where you have a spring fastened in a solid wall. You inject a single pulse from the other side, and it bounces back reversed.
It’s pretty much the same thing with electromagnetic waves.
Non-metals also reflect, although they reflect back certain colours. For example, a black t-shirt absorbs all the wavelengths (all teh “types” of photons), and relects nothing back. A blue t-shirt absorbs everything, except for the blue wavelength which gets reflected out, and we are able to perceive it. All of this has to do with the types of molecules present in the material, and at what wavelengths they absorb light (some molecules also emit light, or flouresce, but I’m not going to go into that). The molecules absorb in such ways as to cause changes in molecular rotation, vibration, or even electronic or nuclear states, and since quantum mechanics dictates what happens, only certain wavelengths of energy get absorbed. The rest passes right on through, or gets reflected.
I know i skimmed over stuff a little, but I’m tired and I don’t really have time to be answering this post, let alone in detail 
Others with more knowledge will without a doubt come along soon.
…
Did you just post that to keep the question at the top of the list?
Ok ok, i know the stuff abt photons being absorbed on dark material but i still dont get the “hows and whys”.
Thanks for the contributions anyway.
And yes, … was to keep the thread up;)