"Speed of light and Relativity- Do they hold in any media?" If I believe correctly, one of major considerations in relativity is 'c', being constant speed of light in vacuum. But, velocity in any media is different than vacuum for light. Speed in water is almost 75% of c.(Jean Foucault in 1850) now, v/c is refrective index of medium and is less than unity. Also the medium turns the light's direction while change in media. One more fact, Cherenkov effect, shows electrons in heavy water pool of pool type nuclear reactor travelling faster than speed of light. This tends to emission of photons. Now, this makes a series of questions in my mind: 1. Does laws of relativity hold for vacuum only? 2. How can law be established if vacuum is never measured to be perfact, u can just decide it to be almost zero probability of finding any atom in media. 3. What changes velocity of light in a media? 4. What do u predict for photons in above case coming out of even faster electrons? Does the inertia laws apply to them? read the effect in detail to know about emission's nature and suggest me the answer.