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The speed of light in a vacuum (which is constant for all observers: see the [[Theory of Relativity|theory of relativity]]) is, to nine significant figures, exactly 299,792,458 metres per second (approximately 1,079,252,848 km/h). As the speed of light is used to define the [[SI]] metre, this is the value ''by definition''.

The speed of light in a vacuum (postulated to be constant for all observers by the [[Theory of Relativity|theory of relativity]]) is, to nine significant figures, exactly 299,792,458 metres per second (approximately 1,079,252,848 km/h). As the speed of light is used to define the [[SI]] metre, this is the value ''by definition''.

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When overtaken by a charged particle in a vacuum, it emits [[Cherenkov Radiation]].

The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of [[quantum mechanics]], particularly the concept of a path of least action) the phenomenon of [[refraction]]. When overtaken by a charged particle in a vacuum, it emits [[Cherenkov Radiation]].