Light travels at a speed of 299,792.5 kilometers per second (186,282 miles per second), and nothing can go faster than that according to special relatively (see 1905). However, light travels more slowly when passing through matter, and the decrease in speed is more marked as the index of refraction of the transparent medium increases. In water, light travels at 224,900 kilometers per second, and only 124,000 kilometers per second in diamond. The speed of light is even a bit below maximum when it is passing through air.
A rapidly moving particle can never travel faster than the speed of light in a vacuum, but it can travel faster than light does in water, let us say. And if the particle is moving extremely close to the speed of light in a vacuum, it may move faster in air than light does. When particles move faster than light in some medium other than a vacuum, they leave a wake of light trailing behind.
A Soviet physicist, Pavel Alekseyevich Cherenkov (b. 1904), was the first to observe this wake of radiation, which came to be called Cherenkov radiation as a result. The reason for the wake was explained by the Russian physicists Igor Yevgenyevich Tamm (1895-1971) and Ilya Mikhaylovich Frank (b. 1908).
From the angle at which Cherenkov radiation is emitted, the speed of ultrafast particles can be calculated, and for this finding, Cherenkov, Tamm, and Frank were awarded the Nobel Prize for physics in 1958.
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|Publication:||Asimov's Chronology of Science & Discovery, Updated ed.|
|Article Type:||Reference Source|
|Date:||Jan 1, 1994|
|Previous Article:||Artificial radioactivity.|