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As Cherenkov radiation passes through the water, the charged particles travel faster than light can through that medium. So, the light you see has a higher frequency (or shorter wavelength) than the usual wavelength. Because there is more light with a short wavelength, the light appears blue. But, why is there any light at all? It's because the fast-moving charged particle excites the electrons of the water molecules. These electrons absorb energy and release it as photons (light) as they return to equilibrium. Ordinarily, some of these photons would cancel each other out (destructive interference), so you wouldn't see a glow. But, when the particle travels faster than light can travel through the water, the shock wave produces constructive interference that you see as a glow.”

Oskarshamn Reaktor 3 (photo: OKG)

Clab, Oskarshamn (photo: Curt-Robert Lindqvist)

Excerpt taken from Wikipedia “Cherenkov radiation“

“Why Water in a Nuclear Reactor Is Blue

Excerpt taken from ThoughtCo.
“Why Is the Water Blue in a Nuclear Reactor? - Cherenkov Radiation”
by Anne Marie Helmenstine, Ph.D. May 20, 2017

Photo: OKG

Why Is the Water Blue in a Nuclear Reactor?

"Cherenkov radiation"..."is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to Cherekov radiation."