The **speed of light** in the vacuum of free space is an important physical constant usually denoted by the letter *c*.

## Explained[]

It is the speed of all electromagnetic radiation, including visible light, in free space. Further, it is the speed of anything having zero rest mass.

### Definition of SI metre[]

The SI metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second.

#### Explained[]

The speed of light can be assigned a definite numerical value because the fundamental SI unit of length, the metre, has been defined since 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second; in other words, any increase in the measurement precision of the speed of light would reduce the uncertainty in practical realizations of the metre but would not alter the numerical value of *c*.

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum.

### Refractive index[]

The ratio of the speed of light in the vacuum to the observed phase velocity is called the refractive index of the medium.

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