Super-Kamiokande is a water Cherenkov detector. In water, light travels
about 25% slower than it does in a vacuum and it is possible for an energetic
particle to travel faster than light. (This is not possible in the vacuum
where light is the fastest things there is!). When a particle travels faster
than the speed of light in water, it produces a shock wave that is the
equivalent of a sonic boom made by a jet travelling faster than the speed
of sound in air. This shock wave takes the form of blue light called Cherenkov
light, after the Russian physicist Pavel Cherenkov,wall of the detector.
This light is detected by an array of light sensitive photomultipliers,
as illustrated below. The image is in the form of a ring (red tubes).
By measuring the brightness, shape, and direction of the ring we can
figure out how much energy the particle had, whether it is a
muon or electron, and which way it was going.