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 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.

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. This means it took light from the galaxy 2.5 million years to travel to us, implying that the Universe has to be at least 2.5 million years old in order to allow light enough time to make the journey.

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