![]() In specially-prepared media, the speed can be tiny, or even zero.įor many years the speed of light was the subject of speculation, some believing it to be infinite. ![]() The speed is inversely proportional to the refractive index of the medium. The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in vacuum. In astronomy, distances are often measured in light-years, the distance light travels in a year. 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. An exception is where long distances or precise time measurements are involved. More about this topic is found below in Speed of light set by definition.įor many practical purposes, the speed of light is so great that it can be regarded to travel instantaneously. In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s) because of the way the metre is defined. The speed of light also plays a role in general relativity, and is believed to be the speed of gravitational waves. Speeds faster than that of light are encountered in physics but, in all such cases, no matter or information is transmitted faster than c. The speed of light is the speed of not just visible light, but of all electromagnetic radiation in vacuum (also called free space), and usually is denoted by the symbol c. The term speed of light generally refers to a fundamental physical constant of spacetime that limits the rate of transfer of matter or information.
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