Speed of light

The speed of light in the vacuum of free space is an important physical constant usually denoted by the symbol c0 or simply c. The metre is defined such that the speed of light in free space is exactly 299,792,458 metres per second (m/s).
The speed of light is of fundamental importance in physics. It is the speed of not just visible light, but of all electromagnetic radiation, and it is believed to be the speed of anything having zero rest mass, and of gravitational waves. Einstein's theory of relativity together with the principle of causality requires that no matter or information can travel faster than the speed of light. Speeds faster than that of light are encountered in physics but, in all such cases, no matter or information is transmitted faster than c.
For many practical purposes, the speed of light is so great that it can be regarded as to travel in an instant. An exception is where long distances or precise time measurements are involved. For example, in the Global Positioning System (GPS), a GPS receiver measures its distance to satellites based on how long it takes for a radio signal to arrive from the satellite. In astronomy, distances are often measured in light-years, the distance light travels in a year (around ten trillion kilometres).
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. The speed is proportional to the refractive index of the medium. In specially-prepared media, the speed can be tiny, or even zero.
For many years the speed of light was the subject of speculation, some believing it to be infinite. The first effective measurements of the speed of light were made in the seventeenth century, and these were progressively refined. In 1983, the speed of light in free space was fixed by definition.