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Line 1: − Scientists claim the speed of light to be exactly 299,792,458 metres per second (1,079,252,848.8 km/h).+ + +
impossible to convert a value into a value with more significant figures than the original value or the conversion factor
The speed of light in a vacuum (which is constant for all observers: see the [[theory of relativity]]) is, to nine significant figures, exactly 299,792,458 metres per second (approximately one million 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]].