Cerenkov effect
Cherenkov effect (chə-rĕng′kôf, -kəf)
The emission of electromagnetic radiation by charged particles traveling through a medium at a speed faster than the speed of light in that medium. This radiation is called Cerenkov radiation. When it results from cosmic rays penetrating the atmosphere at high speeds, it generally takes the form of bluish light. This effect is named after its discoverer, Russian physicist Pavel Alekseyevich Cerenkov (1904-1990).
The American Heritage® Science Dictionary
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