What is a photon?
Quick Answer
A photon is the quantum particle of light and other electromagnetic radiation. It has no rest mass and always travels at the speed of light in vacuum.
The Full Story
A photon is the smallest discrete unit, or quantum, of electromagnetic radiation. It carries energy and momentum, and its energy depends on its frequency: higher-frequency photons such as X-rays carry more energy than radio photons. Photons are emitted and absorbed by atoms, which is why light can transfer energy without needing a material medium. The concept grew out of early 20th-century quantum theory, especially the work of Albert Einstein on the photoelectric effect in 1905.
Key Facts
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"The tiny particle that makes up every beam of light"
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