cerenkov-radiation

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Nothing moves faster than the speed of light, unless it’s Cherenkov Radiation. As light travels through a medium (e.g. water, air, not-a-vacuum), it slows down. Then when a particle moves faster than the speed of light in this medium, it creates a sort of visible “sonic boom,” named Cherenkov Radiation.
The images above show nuclear reactors and the Cherenkov Radiation that the fluid around them emits.

From the annals of XKCD-What If?: What if I took a swim in a typical spent nuclear fuel pool?

“For the kinds of radiation coming off spent nuclear fuel, every 7 centimeters of water cuts the amount of radiation in half … outside the [danger zone], you could swim around as long as you wanted—the dose from the core would be less than the normal background dose you get walking around. In fact, as long as you were underwater, you would be shielded from most of that normal background dose. You may actually receive a lower dose of radiation treading water in a spent fuel pool than walking around on the street.”