Speed of light

Speed of light
The speed of light in a vacuum is exactly 299 792 458 m/s (fixed by definition). Although this quantity is sometimes referred to as the "velocity of light", the word velocity refers to a vector quantity, which has a direction (and speed refers to the magnitude of the velocity vector).The speed of light has been measured many times, by many physicists. Though Galileo attempted to measure the speed of light in the 1600s, the best early measurement in Europe was by Ole Rømer, a Danish physicist, in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. If he had known the diameter of the orbit (which he did not) he would have deduced a speed of 227 000 km/s.The first successful measurement of the speed of light in Europe using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313 000 km/s.Léon Foucault used rotating mirrors to obtain a value of 298 000 km/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299 796 km/s. This was close to the modern value of 299 792 458 m/s