Date: Sunday, September 15, 2002
How close must two observers be so both can see a single photon?
A single photon can be observed by only one observer. It is
"observed" when it interacts with something. When it interacts, it
either is changed or gone completely. For example, in the
photoelectric effect a photon strikes an electron, giving it energy
and momentum which can cause the electron to escape and be detected
elsewhere. The photon's energy and momentum is then changed (reduced)
by the energy and momentum given to the electron.
A photon can also be absorbed by a molecule increasing the energy of
the molecule by the photon's energy. This changes the state of the
molecule, which explains where sunburns come from. Molecules in
florescent materials are "excited" (moved to higher energy levels)
when the photons in the light are absorbed. The florescent material
then gives off light as the molecules slowly decay to lower energy
levels, emitting photons.
Best, Dick Plano
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