Don Lincoln tackles the origin of Cerenkov radiation this time. This is the case where a body travels faster than light in a medium.
This is not purely academic. This is how we detect certain particles, such as neutrinos. Those photodetectors in, say, SuperKamiokande, are detecting these Cerenkov radiation. In fact, if you look in a pool of water of nuclear fuel rods, the blue light is the result of Cerenkov radiation.
So here's a chance for you to learn about Cerenkov radiation.