Speed of light

The speed of light in a vacuum is denoted by the letter c for constant or the Latin celeritas (speed). The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium.
In metric units, c is exactly 299,792,458 metres per second or 1,079,252,848.8 kilometres per hour. Converted to imperial units, it is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour. Note that this speed is a definition, not a measurement, since the fundamental SI unit of length, the metre, has been defined since 21 October 1983 in terms of the speed of light—one metre is the distance light travels in a vacuum in 1/299,792,458 of a second.

According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed or velocity in a vacuum, commonly known as the speed of light, which is a physical constant denoted as c. This speed c is also the speed of the propagation of gravity in the theory of general relativity.

One consequence of the laws of electromagnetism is that the speed c of electromagnetic radiation does not depend on the velocity of the object emitting the radiation; thus for instance the light emitted from a rapidly moving light source would travel at the same speed as the light coming from a stationary light source (although the colour, frequency, energy, and momentum of the light will be shifted, which is called the relativistic Doppler effect). If one combines this observation with the principle of relativity, one concludes that all observers will measure the speed of light in vacuum as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light. Because of this, one can view c as a fundamental physical constant. This fact can then be used as a basis for the theory of special relativity. It is worth noting that it is the constant speed c, rather than light itself, which is fundamental to special relativity; thus if light is somehow manipulated to travel at less than c, this will not directly affect the theory of special relativity.