Cherenkov radiation can be said to be the light equivalent of a sonic boom. In a medium such as air or water, the speed of light is considerably less than the speed of light in vacuum (c). An energetic particle enters the atmosphere at c. It's now moving faster than light does in air. A cascade of photons ensues - that's the light shower that the HESS telescope detects. It is important to remember that the cosmic rays are not travelling faster than c. They are just travelling faster than light does after it has been slowed down by passing through a material. Also: The blue glow in the water surrounding nuclear reactors is Cherenkov radiation. For more information visit THIS SITE or HERE.
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