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Name: Clarence
Status: student
Grade: 9-12
Location: CA
Country: N/A
Date: 2/1/2005

Hi, I am a curious 12th grade student. I was searching the Internet and came across "Cherenkov radiation". I looked it up and started to understand the "sonic boom" effect and the interference. This is one of the web sites I went to:

But there is some things I do not understand.

1 How can a charged particle be traveling faster than the speed of light?

2 If it is possible for something to travel faster than the speed of light, what medium would it have to be traveling through?

3 What is an "optically transparent medium"? And finally, how do electrons and positrons relate to the Cherenkov radiation?

A charged particle, say an electron, that is generated -- say in a vacuum -- passes through water shielding the particle accelerator its speed may exceed the speed of light IN THAT MEDIUM, WATER, NOT IN A VACUUM. The charged particle emits the characteristic radiation you have found at the web site you cited. The "faster than the speed of light" must be modified by (in the medium). If instead of an electron, the particle is a gamma ray the decays to an electron/positron pair the momentum of the gamma ray can be determined directly.


What is meant by an "optically transparent medium" is the common definition of the term. It means a medium like water that is essentially transparent to visible light. Of course, no medium is absolutely transparent, there is always some scattering of the photons.

Vince Calder

Two things are meant here by the "speed of light":

1) the rate at which light propagates in vacuum. This is the speed that no particle or information transfer can exceed (we think)

2) the rate at which light moves through a volume of space that includes charged particles. (Anything you can touch fits this description.) This speed can be slower that the speed of light in vacuum because it includes effects of light interacting with those charged particles -- being absorbed and re-radiated, etc.

An optically transparent medium is a material that is transparent to visible light -- i.e., anything you can see through.

Tim Mooney

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