John, Colchester asked:
What would happen if you put a very strong magnet behind the mirror? Would this cause the light to go out of focus?
I’m not sure that it would go out of focus but something it will definitely do is affect the polarisation of the light. Polarisation, if you imagine light’s a wave: if you imagine a wave on a tightrope - vibrate it up and down that would be vertically polarised. If I vibrate it from side-to–side and it would be horizontally polarised. The light will hit the mirror as an electric current flow (see this question) – what magnets do to an electric current is cause them to bend and move in circles. Because the magnetic field will cause the electrons to bend when the light hits it the reflection will have a slightly twisted polarisation to the one which came in.