speed of light

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speed of light

physics

Also known as: c, electromagnetic constant, velocity of light

Written and fact-checked by The Editors of Encyclopaedia Britannica

Last Updated: Feb 15, 2025 • Article History

Key People:: A.A. Michelson; James Bradley; Harlow Shapley; Lene Hau; Louis Essen

Related Topics:: light; physical constant; velocity; E = mc2

On the Web:: CORE - Investigation of the Effect of Density on the Speed of Light Travelling Through a Medium (PDF) (Feb. 15, 2025)

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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.

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 = mc², 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.