How do you know the speed of light?

Answer 1

The speed of light in air was first accurately found using earthbound equipment (as opposed to astronomical techniques) by Hippolyte Fizeau in 1849, using ideas that were mooted many years before by carious scientists - including Galileo. in Fizeau's experiment a beam of light was directed at a mirror several thousand metres away. On the way from the light source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. The whel was manipulated until the beam was detected both on the outward and return journey. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Using this method Fizeau reported the speed of light as 313,000 kilometres per second. In 1926, Michelson used a rotating prism to measure the time it took light to make a round trip from Mount Wilson to Mount san Antonio in California USA, a distance of about 22 miles (36 km). The precise measurements obtainable from this more sophisticated arrangement yielded a speed of 186,285 miles per second (299,796 kilometres per second), very close to the accepted figure today - a figure that is obtained by much more sophisticated and aurate techniques.

Don't forget, though, that the speed of light will vary considerably depending upon what medium the light is passing through. The above figure represnted the speed of light in air; in a vacuum it is slightly faster, but in dense media like glass or quartz, the speed of light is considerably reduced.