Speed of light

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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 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.

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 (6x10^12) 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.

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