The distinctive blue hue from Cherenkov radiation produced by a nuclear reactor's core.
An Optical Sonic Boom -- the Cherenkov Radiation
While nothing can outrun light in the vacuum of space, certain particles can edge out light in a short race inside water. Because light only travels at roughly 75% of its "top speed" inside water, it is possible for highly energetic particles to outrun light waves. In the case of a charged particle such as an electron, this results in a distinctive blue flash analogous to the sonic boom generated by a fighter jet when it exceeds the speed of sound. This distinctive blue flash is known as Cherenkov radiation.
Argonne National Laboratory
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