Speed of light

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The speed of light in vacuum is a universal physical constant denoted by c and defined exactly as 299,792,458 metres per second. The metre is defined using this fixed value as the distance light travels in vacuum during a specified fraction of a second.

All electromagnetic radiation travels at c in vacuum, including radio waves, visible light, X-rays, and gamma rays. Gravitational waves and other massless field disturbances also propagate at this invariant speed.

In special relativity, the measured value of c is the same for all inertial observers, regardless of the motion of the source or observer. It is also the upper limit at which information, energy, or matter can be transmitted locally through space.

Light travels more slowly through transparent materials because it interacts with the medium. The ratio between c and the propagation speed in a material is its refractive index, and the finite speed of light creates measurable delays in astronomy, communication, navigation, and computing.

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• Source / publisher: Wikipedia
• URL type: WWW
• Credits: Wikipedia
• URL: https://en.wikipedia.org/wiki/Speed_of_light