Here's something I just wrote up. It's a bit 'heretical', so I though I'd
post it here for comments (and to stir up some discussion while things are
so quiet) ;)
It's aimed at pre-highschool, but I might need to simplify it some more.
LASERS EMIT COHERENT LIGHT, BUT NOT BECAUSE THE ATOMS EMIT
IN-PHASE LIGHT WAVES
Even fairly advanced textbooks fail to give the real reason why laser
light is coherent. It is true that the flourescing atoms in a laser all
emit light that is in phase with the waves already travelling between the
mirrors. But in-phase emission creates amplification, it does not create
coherent light. If you were to feed incoherent light into a laser, the
atoms would emit in-phase waves, and the laser would amplify the light.
But the brighter light would still be incoherent! How did the light
within the laser get to be coherent in the first place?
Lasers create coherent light because of their mirrors. Imagine a
simplified laser having flat, parallel mirrors. As light bounces between
the mirrors, the light thinks that it's travelling down an infinitely long
"virtual tunnel". (Have you ever held up two mirrors facing each other?)
When first turned on, the laser emits incoherent light. After a few
thousand mirror bounces, all the different waves which started out from
different parts of the laser tube have added and subtracted to form a
single wave. A similar thing happens with starlight: starlight is
coherent, even though sunlight is not. Laser light is coherent because
the bouncing light has travelled millions of miles between mirrors, and
the various competing waves have melded together to form a single pure
wave.