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The speed of light in a vacuum is, to nine significant figures, 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 constant for all observers (see the [[Theory of Relativity|Theory of Relativity]])
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 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]].
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]].
Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is know as a [[light-year]], which is about 6 million million (6x10<sup>12</sup>) miles. For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away.
Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is know as a [[light-year]], which is about 6 million million (6x10<sup>12</sup>) miles. For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away.
The finite speed of light raises a severe problem with reconciling cosmological observations with a young (~6000 year) Universe. See the [[starlight problem]].
The speed of light raises several questions in physics including that of the [[starlight problem]].