Measurement the Speed Light

The speed of light in a vacuum (in free space) is an important physical constant usually denoted by the symbol c. It is defined as exactly 299,792,458 metres per second (due to the definition of the metre). According to special relativity, it also is the speed of anything having zero rest mass. To within experimental accuracy it is the speed of all electromagnetic radiation in vacuum, and is anticipated to be the speed of gravitational waves as well.
Einstein's theory of relativity together with the principle of causality requires that no mattrer or information can travel with a speed larger than c. Speeds faster than that of light in a vacuum 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 to travel instantaneously. 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.
The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in free space. The speed is inversely 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. Today measurement of light in vacuum continues, focusing upon setting limits upon any field or frequency dependence. In 1983, the standard for the speed of light in free space (an ideal vacuum) was fixed by definition.