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Speed
of light
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Although some people speak of the "velocity of light", the word velocity
should be reserved for vector quantities, that is, those with both magnitude
and direction. The speed of light is a scalar quantity, having only magnitude
and no direction, and therefore speed is the correct term.The speed of light
has been measured many times, by many physicists. The best early measurement
is Ole Rømer's (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 a speed of 227,000
kilometres per second (approximately 141,050 miles per second).
The first successful measurement of the speed of light 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 kilometres per second.
Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/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 186,285 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.
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