[apochromat]

Sorry to get off-track, but does relativity only apply in a vacuum?

If light is slowed by the medium (enough?), then its relative velocity starts(?) to depend on the observer's speed relative to the medium, or what?

[RightWhale]

does relativity only apply in a vacuum?

Probably not just in a vacuum, but at normal speeds, such as one might find inside a gas, liquid, or solid, the effect of relativity would be small, or in the range where it could be safely ignored. Plasma is nearly a vacuum, so relativity is probably important in that regieme.