Question: If you could have a tin where the inside is perfectly reflective, and you closed the lid, would the light photons remain inside? (a friend brought this to my attention, we both really really want to know!)

Martin Zaltz Austwick answered on 19 Mar 2012:

Yes! They’d just keep bouncing around until you opened the tin again. This is a little bit like how a laser works. The laser beam bounces backwards and forwards between two mirrors. Because of the way a laser works, it gets more and more powerful as it does this, and builds up energy. One of the mirrors is not completely perfect, and lets some light out into the outside world – and that’s the laser beam you see! So the laser beam you see is normally much less powerful than the beam bouncing around inside the laser itself.