If, indeed, the speed of light is constant,why is it usually listed for a vacuum?

Asked by: Doris Thomas

Answer

In a medium other than a vacuum, light propagates by a cascade of interactions with the molecules or atoms that make up the medium. Photons, the 'particles' of light, are absorbed and re-radiated (by atoms or molecules), absorbed again and re-radiated again, and over-and-over as they pass through the medium.

Since each absorption and re-radiation interaction takes time, though it be very little, the cascade cannot travel as fast through a material medium as light can in a vacuum. Light travelling in a vacuum does not have to 'waste time' being repeatedly absorbed and re-radiated. For this reason, the speed of light in a material medium is generally less than c, and will exhibit a specific value that depends upon the characteristics of the medium.

Since a vacuum exists between the atoms or molecules of a material medium, the speed of light between absorption and re-radiation sites is still c. It is the interactions that cause the overall speed through a material medium to differ from the standard value c in vacuum.

Clearly, the speed of light in a vacuum is, in a sense, more 'fundamental' than the speed of light through any particular material medium. For this reason, the speed of light is usually quoted in vacuum. It is also much more convenient for reference purposes to quote the speed of light in a vacuum where the details of the medium, which may not be reproducible to sufficient accuracy from laboratory-to-laboratory, do not have to be considered or dealt with.
Answered by: Warren Davis, Ph.D., President, Davis Associates, Inc., Newton, MA USA