text: The picture which I could see in Wikipedia, after removing the [)] from your link was of Cherenkov Radiation. C.R. happens when light travels faster than it's normal speed IN THAT CERTAIN MEDIUM. The speed of light is not same everywhere. It travels with the fastest speed in vacuum, that is the ultimate speed(3 x 10^8 m/s) of light. In heavy water it travels with a certain slower speed. When light is made to travel faster in heavy water(which is faster than the normal speed in heavy water but still slower than speed of light(c)) C.R is emitted. So, far no one has managed to make light move faster than c.