Speed of light

موضوع: Speed of light الجمعة فبراير 27, 2009 12:18 am

The speed of light in a vacuum is an important physical constantc. The metre is defined such that the speed of light in free space is exactly 299,792,458 metres per second usually denoted by the symbol (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 with a speed larger 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 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 refractive index of the medium. In specially-prepared media, the speed can be tiny, or even zero. material medium, like glass or air, is less than its speed in free space. The speed is inversely proportional to the
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 until, in 1983, the speed of light in free space
was fixed by definition.