ZOOM in and out space earth planet cool sweet science technology cells cell atoms atom The speed of light, usually denoted by c, is a physical constant representing the speed at which light and all otherelectromagnetic radiation travels in vacuum. Its value is exactly 299792458 metres per second[1][2] (approximately 186282 miles per second). In the theory of relativity, c connects space and time, and appears in the famous equation of mass--energy equivalence E = mc2.[3] The speed of light is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity and of 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 ≈ 200000 km/s; therefractive index of air for visible light is about 1.0003, so the speed of light in air is very close to c. Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. After centuries of increasingly precise measurements, in 1975 the speed of light was ...
