Ever since I took GR I have always been interested in how snell's law changes, if it changes at all, in curved spacetime. The index of refraction depends on the optical density of the medium and this would be observer dependent? am i right on that? i would love to see a paper or other online reference where snells law is derived in GR. i.e in the strong field relativistic limit. or maybe it doesnt change at all? it just stays n1 sin[x1 ] = n2 sin[x2] like P = number density* boltzmann's constant *Temperature holds relativistically and non-relativistically as I recall. thanks!