Cerenkov radiation occurs when a particle plows into a substance such as water and momentarily exceeds the speed of light in that medium. That causes the weird blue glow around some nuclear reactors that are water damped.
Then the excessive energy that has to be disposed of during the velocity change generates radiation? Is this true of all mediums? When light transitions from water to air to space does it accelerate? Where does it get the energy to accelerate? Does it absorb it from the medium it is leaving?