[apochromat]

That makes sense, but what should be made of the case where light is slowed down to a fraction of its vacuum speed, say for instance to 20 mph in a BEC medium? As far as I can guess, the speed of such light could be varied by moving the medium while it contains the light.

There would have to be some light speed reduction or medium (or something) that would define the regime of non-relativity, it seems.

[RightWhale]

the case where light is slowed down to a fraction of its vacuum speed, say for instance to 20 mph in a BEC medium?

The Bose-Einstein Condensate is not a place one would normally try to travel in or have a picnic. But perhaps when I am doing my doctoral studies, I will have the opportunity to measure the angle of Cerenkov radiation inside there.