The speed of light (usually denoted c) is a physical constant. Its value is exactly 299,792,458 metres per second,[1][2] often approximated as 300,000 kilometres per second or 186,000 miles per second. It is the speed of electromagnetic radiation (such as radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down
| Speed of light in different units | |
|---|---|
| metres per second | 299,792,458 (exact) |
| kilometres per second | ≈ 300 thousand |
| kilometres per hour | ≈ 1,079 million |
| miles per second | ≈ 186 thousand |
| miles per hour | ≈ 671 million |
| astronomical units per day | ≈ 173 |
| natural units | 1 |
For much of human history, it was not known whether light was transmitted instantaneously or simply very quickly. In the 17th century, Ole Rømer first demonstrated that it travelled at a finite speed by studying the apparent motion of Jupiter's moon Io. By 1975, the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[1][2]
According to the theory of special relativity, c connects space and time in the unified structure of spacetime, and its square is the constant of proportionality between mass and energy (E = mc2).[3] In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space,[4] and it is believed to be the speed of gravity and of gravitational waves.[5][6] It is an upper bound on the speed at which energy, matter, and information can travel,[7][8] as surpassing it would be equivalent to travelling backwards in time;[9] its finite value is a limiting factor in the speed of operation of electronic devices.[10]
The actual 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 very close to c.
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