[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.

[apochromat]

BECs are exotic, but they are what prompted my questions.

I was thinking about Cerenkov radiation for several minutes after I wrote my previous reply, while I was also muddling over whether "nonrelativistic" was the appropriate term for me to have used.

After thinking about it some more, I suppose that such exotic phenomena are relativistic, but their conditions are so complex that naively-applied relativistic transformations won't fit.