Learn more physics!
A strong laser beam shot at a mirror, what will happen- the beam reflects back or the mirror burns?
I guess you mean a very powerful laser. The answer depends on how
perfect the mirror is. If it absorbs a little bit of the light, it can
start to degrade, start absorbing more light, then get fried. On the
other hand, every ordinary laser has some mirrors as intrinsic parts,
and they obviously do not routinely get fried. It all depends on how
much absorption occurs at the mirror and how intense the laser beam is.
One experiment I worked on as a graduate student used a laser and
some mirrors which were kept in a room which was not very well sealed
from the outside. Ants crawled in and one crawled on one of the mirrors
and cooked. This set off the process Mike describes. We had to keep
spare mirrors around for these cases. I suppose just plain accumulation
of dust can have the same effect, which is one reason why people in
laser labs like to keep the air clean.
(published on 10/22/2007)
Follow-up on this answer.