When a charged particle can propagate faster than the speed of light in a medium, a characteristic light emission called the Cherenkov effect occurs.
Such an effect is common in nuclear reactors . Around their nuclei, a faint bluish glow forms, which can be brighter according to the intensity of nuclear fission reactions . The phenomenon is also observed when high-speed cosmic rays enter the Earth’s atmosphere. These particles – which are usually metallic nuclei from the Sun and other stars, but also from novae and supernovae (stars in late stages) – commonly arrive at us with speeds very close to the speed of light. During its entry into the upper layers of the atmosphere, its speed is greater than that of light itself.
Importance of refraction for the Cherenkov effect
The speed of light in a medium is measured by its refractive index, a physical property determined by the ratio between the speed of light in a vacuum and the speed of light in the medium. Electromagnetic radiation has its greatest speed in a vacuum. Therefore, the absolute refractive index (between vacuum and medium) will always have values greater than 1 . The refractive index can be calculated using the following ratio:
n = c
If the refractive index of a medium is very high , the speed of light inside it will be much less than the speed of light in a vacuum. In this case, we say that the medium has high refringence , characteristic of some materials such as diamond . Its refractive index reaches 2.4 – this means that, in a vacuum, light travels 2.4 times faster than inside a diamond.
How does Cherenkov radiation work?
The answer to this question won a Nobel Prize for three physicists in 1958 , among them its discoverer, the Russian Pavel Cherenkov . Similar to the shock waves formed by supersonic aircraft , which arise because the aircraft moves faster than the sound disturbance generated by itself, the electrons emitted by nuclear reactions, in media such as water, move faster than the light. Thus, because of this slowdown, they produce a predominantly blue -toned light .