Special & General Relativity Questions and Answers
If the speed of light is a constant, why is it slower in glass and water?
The constancy of the speed of light is only a 'law' within media of the same type and 'index of refraction'. The lowest possible index of refraction is that for space and equals 1.00000 and in which light has its maximal velocity. If you were an observer inside some other medium, then so long as you stayed within that medium, your physics would obey the principles of special relativity. Since the speed of light in a vacuum is higher than that in water or glass, you have the possibility of a phenomenon exceeding the speed of light in water, but not in vacuum. Charged particles that do this produce Cerenkov Radiation as the particles move faster than the speed of light in water.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.