Why is cherenkov radiation blue

In the field of radioactivity, the particles concerned by the Cherenkov effect are electrons and positrons. Very light, they reach very high speeds. An electron must have an energy greater than keV to go faster than light in water. This condition is often fulfilled by beta electrons of radioactivity, but never with the heavy and too slow alpha particles.

During their journey, the corpuscles ionize and rattle the many atoms and molecules they encounter. The restoration of the medium is accompanied by the emission of de-excitation photons, for instance in water of visible photons of blue light. The emission of these photons does not cost much in energy: 2. A keV , eV electron that would spend all its energy in emitting blue photons would produce , of them!

The path of the beta electrons in water is of the order of a few millimeters. On this very short course, the emitted photons are therefore very numerous. While no particle can exceed the speed of light in a vacuum 3. The speed of light in a particular medium is related to the speed of light in a vacuum and the refractive index of the medium. Most Cerenkov radiation is in the ultraviolet spectrum, but part of the energy is visible light and is seen as a blue glow.

Water is intrinsically blue because of its selective absorption of the red end of the spectrum. One explanation of the Cerenkov effect in water is that the atoms in the water become excited by the Cerenkov shock wave and then de-excite, emitting blue light.

Another possible explanation is that the number of photons emitted by such a charged particle is inversely proportional to wavelength. This would mean that more photons are emitted with shorter wavelengths, thereby moving the spectrum to the blue side. Cerenkov radiation can be used to facilitate the detection of low concentrations of biomolecules. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.

Facebook Facebook Twitter Twitter. Updated January 22, Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Cherenkov Radiation. Radiation in Space Gives Clues about the Universe. Do Radioactive Elements Glow in the Dark? Photoelectric Effect: Electrons from Matter and Light. Aether Definition in Alchemy and Science. Visible Light Definition and Wavelengths. Your Privacy Rights. Change institution.

Rent or Buy article Get time limited or full article access on ReadCube. References 1. Close banner Close. Email address Sign up. Get the most important science stories of the day, free in your inbox.