The blue glow is called Cherenkov radiation, and it's caused by very high energy electrically charged particles, such as electrons, traveling faster than the speed of light in a medium, such as water. Think of it like a sonic boom for light. In certain mediums, the speed of light is slower, and particles can travel faster than light in that medium. For example, the speed of light in water is 225,000,000 metres per second, compared to 299,792,458 metres per second in a vacuum, or about 75% the sp