Mirrors are never perfect. They always absorb a little of the light that hits them. Extremely high quality mirrors can let light bounce off them 100 times or so before it's mostly absorbed. Since light travels very fast, it hits the walls of a little box very frequently. It's hard to trap light this way for even a millionth of a second.
On waves: light consists of electromagnetic waves. Everything about it travels in waves. I suspect I'm missing your question somehow, so feel free to follow up.
Mike W.
(published on 07/08/2008)
