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photoelectric effect
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photoelectric effect:

The process by which certain substances give off electrons when exposed to light. A photoelectric cell (which uses the photoelectric effect) varies the flow of current with the amount of light that it receives. Photoelectric cells are used to detect light, as in electric eyes; to measure light, as in light meters in cameras; and as power sources in solar cells in many different devices. Albert Einstein received the Nobel Prize for physics in 1921 for his explanation of the photoelectric effect.