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unit: m/sec
The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Light and all other forms of electromagnetic radiation always travel at this speed in empty space (vacuum), regardless of the motion of the source or the inertial frame of reference of the observer. Its value is exactly 299,792,458 metres per second[1] (approximately 186,282 miles per second). In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass energy equivalence E = mc2.[2] It is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the gravitational waves and an upper bound on the speed at which energy, matter, and information can travel. The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s slower than c. source: http://en.wikipedia.org/wiki/Speed_of_light