On Sun, 26 Mar 2000, Stirling Westrup wrote:
> So, one molecule of gas is slammed into by a photon moving at lightspeed,
> and absorbs it. This gives the molecule a kick in the direction that the
> photon was moving.
I'm not so sure, infrared photons add to the vibrational energy of the
molecule. I only think you will get momentum transfer if the photon
is reflected (as in the case of a light sail). Which raises an
interesting point, if the photons are always going light speed,
and they are transfering momentum to the light sail, then, *shouldn't*
the photon being reflected back be of a lower frequency (to allow
for conservation of energy)? Hmmmmm...
> Now, not only will the molecule be moving faster, but
> it will be in an energized (excited) state, and will at some point in the
> near future fire off a photon in some (usually random) direction, so that
> it can return to its normal state. Of course, when it fires off a photon,
> it recoils and slows down, changing direction as it does.
>
The whole point of why you see color due to the fact that
absorbed photons, *don't* get reemitted (except perhaps a lower
frequency as heat). What you are describing seems more like what occurs
in lasers, when the photons passing through the material stimulate
excited atoms to emit photons of a similar frequency.
I would agree that photons, when absorbed by a gas, increase
the energy state of the molecules, but this can be in
vibrational or rotational modes in addition to velocity.
Most of the time, I don't think the photons are going to be
re-emitted but instead, the increased pressure of the gas will
transfer momentum to the walls of the container which will then radiate
infrared photons as heat. In the upper atmosphere, I suppose
that you would see gas molecules functioning as a black body
and radiating infrared photons.
Robert
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