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Re: Laser light (fwd)



--- FORWARDED MESSAGE ---
On Thu, 5 Dec 1996, David Winsemius <dwinsemius@imagine.com> wrote:

I must say I have enjoyed you Web offerings immensely. I also think
you take scientific argument seriously and will probably be able to
change your position when you are wrong. The paragraph below is, I
believe, in serious error. I hope you will hear me out.

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.

David Winsemius, MD, MPH (AB Physics, Berkeley, '71) not that the
degrees should affect the validity of my arguments.