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billb@eskimo.com wrote:
LASER LIGHT IS NOT "IN PHASE" LIGHT
When light waves of various phases are combined, they inextricably
add together. When the light within a laser causes atoms to emit light
in phase with the stimulating beam, the result is not "in phase"
light, the result is a single, more intense, amplified wave of light.
Single waves are always in phase with themselves, but it's misleading
to imply that a single wave is something called an "in phase" wave.
Laser light could more accurately be called "single wave" light, or
"pointsource" light. The physics term for this is "spatially coherent"
light. Light bulbs, flames, the sun, etc., emit "extended source"
light. Starlight and the light from arc welders is "pointsource" light
and is quite similar to laser light; both of these give light which is
highly spatially coherent.
Laser light is different because it is emitted from an excited
population of electrons which are "in phase" because the electrons
which were in a metastable state have been decayed in sync with the
advancing wave train.
Since free photons are wave-particles (not
waves) I believe you are propagating the incorrect Huygens model by
referring to the light beam as though it is this homogeneous.
The fact
that it is emitted from a point source is also not what distinguishes
lased light from incoherent light. If you finely collimate an
incoherent light source you do not make it coherent, even though you
make it a point source.
Lased light is phase coherent even when it is
not from a point source.