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...

A. assuming the light source was perfectly aligned, so that the light
strikes the surfaces at right angles with no variation - bouncing back
and forth, over lapping itself.

B. the initial source of light is removed quickly so that the first
reflection/bounce doesn't go back into the source thus
terminating/absorbing the light. When I say quickly... I assume it
would have to be faster than the speed of light to be removed before
the light returned back up to reach it?

C. the reflective materials contain no imperfections, as the light
reflects back towards the other it is not absorbed or diminished.

So you have two materials with a beam of light moving between
them...over and over and over...

Would it stay the same temperature?
Would it increase in brightness as the light overlaps itself?
Would it eventually destroy the two reflective materials?

After several gazillion overlaps of the light - how dangerous would it
be if one of the two reflective surfaces was removed and the
intense/over lapped light was allow to escape?

WHY do I think of these scenarios?
(Don't bother answering that last one GRIN).
Cheers.