**Numerical Value, Notation, and Units**

The speed of light in vacuum is denoted * c*. The symbol

*c*is a "

*c*onstant" in physical unit systems, and

*c*also stands for "

*celeritas*", Latin for "swiftness". (Capital

**C**is the SI unit for Coulomb of electric charge.) Originally, the symbol

*V*was used for the speed of light, introduced by James Clerk Maxwell in 1865. In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used

*c*for a different constant later shown to equal √2 times the speed of light in vacuum. In 1894, Paul Drude redefined

*c*with its modern meaning. Einstein used

*V*in his original German-language papers on special relativity in 1905, but in 1907 he switched to

*c*, which by then had become the standard symbol.

Sometimes *c* is used for the speed of waves in *any* material medium, and *c*_{0} for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as other related constants: namely, *μ*_{0} for the vacuum permeability or magnetic constant, *ε*_{0} for the vacuum permittivity or electric constant, and *Z*_{0} for the impedance of free space. This article uses *c* exclusively for the speed of light in vacuum.

In the International System of Units (SI), the metre is defined as the distance light travels in vacuum in 1 ⁄ 299,792,458 of a second. This definition fixes the speed of light in vacuum at exactly 299,792,458 m/s. As a dimensional physical constant, the numerical value of *c* is different for different unit systems. In branches of physics in which *c* appears often, such as in relativity, it is common to use systems of natural units of measurement or the Geometrized unit system where *c* = 1. Using these units, *c* does not appear explicitly because multiplication or division by 1 does not affect the result.

Read more about this topic: Speed Of Light

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