The **speed of light** in vacuum, commonly denoted **c**, is a universal physical constant important in many areas of physics. Its value is exactly **299,792,458 metres per second**, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. This is approximately 186,282.4 miles per second, or about 671 million miles per hour. According to special relativity, *c* is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed at which all massless particles and associated fields (including electromagnetic radiation such as light) travel in vacuum. It is also the speed of gravity (i.e. of gravitational waves) predicted by current theories. Such particles and waves travel at *c* regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, *c* interrelates space and time, and also appears in the famous equation of mass–energy equivalence *E* = *mc*2.

The speed at which light propagates through transparent materials, such as glass or air, is less than *c*. The ratio between *c* and the speed *v* at which light travels in a material is called the refractive index *n* of the material (*n* = *c* / *v*). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at *c* / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is 1.000293, so the speed of light in air is 299,705 km/s or about 88 km/s slower than *c*.