"One way mirrors" are used in interrogation rooms etc. The wikipedia article states that:
A true one way mirror does not, and cannot, exist. Light always passes exactly equally in both directions. However, when one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side.
Which I think I understand. What I find strange is that this surely can't be any deep principle of physics, as it's easily "violated"; you could just place a camera on one side and a TV on the other side, and it would be perfectly "see-through" from one side but not the other.
Couldn't you imagine a sort of microscopic version of the above "violation"?
