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Re: Photons



How come a photon can pick up another photon (of same frequency) stored in
an excited electron orbiting a nucleus (induced transitions). How come the
stored photon gets picked-up. What would be wrong if it did not happen.

I'm not sure what you are getting at with this question. As Ludwik
pointed out, it doesn't make much sense to talk about "stored"
photons. Photons are best thought of as being created and annihilated
at the beginning and ends of their journeys through space-time.

If, as seems likely, you are wondering why the emission of a photon
from a suitably prepared atom is stimulated by the passage of a
photon of similar ilk, one can do a fairly simple energy-momentum
conservation calculation, along the lines of a one-dimensional
Compton effect analysis, and see that a photon incident upon an atom
in an appropriately excited state can only conserve both energy and
momentum if the atom emits a new photon of the same energy as the
incident one and in the same direction. That the phase has to be the
same is not so easily shown, but an arm-waving argument has to do
with the fact that the process takes a very short time and so the two
photons "should" come away from the encounter with the atom with the
same phase.

Of course, because these photon-atom collisions are random events,
this phase will drift with time (relatively slowly), and a laser is
characterized by, among other things, its "coherence length," which
is the length of the moving beam over which the photons are pretty
close to being "in phase." When doing holography, one has to be
careful to consider this value, since if the reference beam and the
reflected beams have a path length that differs by more than the
coherence length you will not get a hologram. Fortunately, this
length is usually on the order of centimeters, so it is easily
allowed for. But remember, 3 cm represents only about 10^-10 s, so
the time that a typical coherence length represents is only a few
times 10^-10 s.

Hugh
--

Hugh Haskell
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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