Originally posted by DeepThought
If they are perfect mirrors then the room will remain illuminated.
It is very difficult to make perfect mirrors. The best mirrors rely on total internal reflection. So maybe the best experiment would be an optic fibre ring. There is of course some loss even with optic fibres, so the question is, how long would light last in a ring of optic fibre. My guess is not very long at all. I believe that an optic fibre from Cape Town to Europe carries the signal in about 0.3 of a second, and some of that is the devices on the way. And I believe there is some loss over that distance.
So my guess would be that light in an optic fibre ring, or very long optic fibre would all be lost in about a minute.
I believe lasers also use mirrors with light bouncing back and forth.