Speed of light
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The speed of light in the vacuum of free space is an important physical constant usually denoted by the letter c. It is the speed of all electromagnetic radiation, including visible light, in free space. It is the speed of anything having zero rest mass. The SI metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h). The speed of light can be assigned a definite numerical value because 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; in other words, 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 c. The approximate value of 3×108 m/s is commonly used in rough estimates (the error is 0.07%). In imperial units, the speed of light is about 670,616,629.4 miles per hour or 983,571,056.4 feet per second (roughly one foot per nanosecond), which is about 186,282.397 miles per second.
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 the speed of light in the vacuum to the observed phase velocity is called the refractive index of the medium. See dispersion (optics). In general relativity c remains an important constant of spacetime, however the concepts of ‘distance’, ‘time’, and therefore ’speed’ are not always unambiguously defined due to the curvature of spacetime caused by gravitation. When measured locally, light in a vacuum always passes an observer at c.