"...while light of any wavelength might slow down instantaneously in a medium, anything with mass won’t. So if you’re moving at 80%, 90%, or 99.9999% the speed of light-in-vacuum and then you transition into water all of a sudden, you’ll find yourself moving faster than the speed of light in that new medium! And if that’s the case, something remarkable happens.
When a charged particle moving slower than the speed of light-in-vacuum enters a medium where it’s moving faster than light in that medium, it polarizes the surrounding molecules as it passes them. The excited molecules quickly drop back down to their ground state, emitting a very special type of radiation known as Čerenkov radiation, which shows up as a characteristic blue glow in, for example, the nuclear reactor shown above."
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