Speed of light | Definition, Equation, Constant, & Facts | Britannica (original) (raw)

Science Physics Matter & Energy

speed of light, speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second.

relativistic velocity combinationIf you race at a beam of light, why doesn't the light approach you faster than the speed of light? If you run away, why doesn't the light approach you slower than the speed of light? This video is an episode in Brian Greene's Daily Equation series.

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The speed of light is considered a fundamental constant of nature. Its significance is far broader than its role in describing a property of electromagnetic waves. It serves as the single limiting velocity in the universe, being an upper bound to the propagation speed of signals and to the speeds of all material particles. In the famous relativity equation, E = m _c_2, the speed of light (c) serves as a constant of proportionality, linking the formerly disparate concepts of mass (m) and energy (E).

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Adam Augustyn.