Re: What factors affects the speed of light?

Hello, Andreas:

    The speed of light in vacuum is a constant of nature. No one really
knows why it has exactly the value that it does, although if it were very
much different, the universe as we know it could not exist (and therefore
we couldn't either).

    The speed of light in materials is described by the index of 
refraction of the material (which itself depends on the frequency of the
light). It is always slower than the speed of light in vacuum, and is 
given by c/n, where c is the speed of light in vacuum and n is the index 
of refraction, measured at the frequency of the light (so n is always
greater than 1). This speed is constant *relative to the material*, so 
that if you are moving with respect to the material (or the material is
moving with respect to you), this needs to be taken into account, making
sure that you add the speeds in the correct relativistic manner.

    Note that the limitation on the motion of objects (or information) is
the speed of light in vacuum; since light moves more slowly through a
material medium than through vacuum, it is possible for objects (typically
elementary particles) to move more quickly through a material than light
does. Such objects, if electrically charged, emit a kind of radiation
called Cerenkov radiation, which is the electromagnetic analogue of a sonic
boom.

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