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Cherenkov radiation happens when electrically charged particles, such as protons or electrons, travel faster than light in a clear medium like water. When this happens, the water molecules and particles interact to give off light.
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If you’ve ever seen photos of a nuclear reactor, then you might notice a blue glow surrounding the core.  

This phenomenon is called Cherenkov radiation, which is essentially a shockwave of light! 

Cherenkov radiation was first discovered in 1934 by Russian physicist Pavel Cherenkov and later expanded upon by Ilya Frank and Igor Tamm. The three men won a Nobel Prize in 1958 for their work to demonstrate and explain how Cherenkov radiation actually works.

How does Cherenkov Radiation Work? 

Cherenkov Radiation captured inside the Advanced Test Reactor at Idaho National Laboratory.
Cherenkov Radiation captured inside the Advanced Test Reactor at Idaho National Laboratory.
Idaho National Laboratory

Cherenkov radiation happens when electrically charged particles, such as protons or electrons, travel faster than light in a clear medium like water. When this happens, the water molecules and particles interact to give off light. 

How is it possible to travel faster than the speed of light? 

Light slows down to 75 percent of its normal speed when it travels through water. This allows the particles emitted from nuclear fuel to move faster than light in water.  

As these charged particles disrupt water molecules in their path, light particles, known as photons, are released — creating a visible “shockwave” of blue or violet light.  

This is similar to the sonic boom when objects move faster than the speed of sound.  

Perhaps you’ve witnessed this phenomenon when a plane zips by and the sound follows.

Why is Cherenkov Radiation Blue?

Comparison of wavelength, frequency and energy for the electromagnetic spectrum.
Comparison of wavelength, frequency and energy for the electromagnetic spectrum.
NASA's Imagine the Universe

Photons are a type of electromagnetic radiation that travel in waves. The wave forms are defined by the height and distance between them.   

The photons resulting from Cherenkov radiation have a high frequency and short wavelength. This appears to the human eye as blue or violet on the electromagnetic spectrum.

The light intensity can be used for accounting of nuclear material at nuclear facilities, conducting physics experiments to determine the energy and trajectory of subatomic particles, and for helping to study and detect cosmic showers in outer space.