Quote:
Originally Posted by absael
I saw a post here in which the poster seemed to be saying that the speed of light is less when traveling through matter. I have also seen numerous Internet sites that seem to say this. My understanding is that the speed of light is constant, but that it propagates through matter at different rates as atoms absorb and re-emit photons. This seems to me to be an important distinction. Which is correct?
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The speed of light is constant in a vacuum, but transits matter in the process you described. The actual speed of light in the matter (any dielectric) remains c between the atoms, but the constant absorption and re-radiation introduces delays at the atoms. The net effective velocity is therefore less than c, and it equal to the refractive index times c, where the refractive index is the square root of the relative permitivity of the material divided by the relative permeability.