> The speed of light is not constant, the speed of light in a vacuum is.
> Light, just as all electromagnetic waves, has a speed that is related to
> the electric and magnetic permeability of the medium that they are
> traveling through.  For example.  Light travels slower through glass
> than air, although, not really by that much.  A better example would be
> to say something like FM Waves (which travel at the speed of light in a
> vacuum) travel slower in magnetic materials than they do in air.

Actually the speed of light is the ideally highest speed attainable by any
particle known (publicly ?). To satisfy the equation E=m*c^2 with E
constant obvously m~=0. The photon satisfies this condition in a vacuum.
The lower speed of photons in other mediums than vacuum is affected by the
interaction of the photon with the mass and other forces present in the
medium. This is what causes it to 'slow down'. Basically the presence of
the medium (as opposed to the emptiness of vacuum) affects the probability
of the photon to be a particle/wave so it interacts to a certain extent
with the material (partly as a wave and partly as a particle).

Peter

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