wrote:
Ok here's another thought, if you had two reflective surfaces facing
each other in parallel... and somehow you were able to shine a light
between them...


Tut writes:

Basically, you've described the thought process that lead to the ruby
laser. Sam Goldwasser has written much that's understandable on the
subject--recommend you google upenn.edu+sam to get you into his FAQ
area.

Good Luck

Tut