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The speed of light in a vacuum is an important physical constant denoted by the letter c0 for constant or for the Latin celeritas ("swiftness").[1][2] It is the speed of all electromagnetic radiation, including visible light, in a vacuum. More generally, it is the speed of anything having zero rest mass.

In metric units, the speed of light in vacuum is exactly 299,792,458 metres per second (1,079,252,848.8 km/h).[3] The fundamental SI unit of length, the metre, has been defined since October 21, 1983, as the distance light travels in a vacuum in 1/299,792,458 of a second; any increase in the measurement precision of the speed of light would refine the definition of the metre, but not alter the numerical value of c0. The approximate value of 3 × 108 m/s is commonly used in rough estimates. In imperial units, the speed of light is about 670,616,629.2 miles per hour or 983,571,056 feet per second, which is about 186,282.397 miles per second, or roughly one foot per nanosecond. See also the later section of this article at "Speed of light set by definition".

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 ratio of c0 to the observed phase velocity is called the refractive index of the medium. General relativity explains how a gravitational potential can affect the apparent speed of distant light in a vacuum, but locally light in a vacuum always passes an observer at a rate of c0.