Speed of Light

The first estimate of the speed of light was made in 1675 by Danish astronomer Ole Roemer. Roemer had been recording the movement of Jupiter's moons when he noticed that the times they appeared and vanished seemed to vary throughout the year. He guessed that this was because the distance from the earth to Jupiter changed during a yearly cycle and therefore so did the distance that the light had to travel. By simple mathematics, Roemer estimated that the speed of light was 200,000 km per second. The first land based estimate was not made until 1849 by Armand Fizeau. Using a fast turning toothed wheel, he timed a beam of light as it traveled toward a mirror 9 km away and was reflected back again. On its outward journey, the light passed between two of the wheels teeth. If the wheel turned fast enough, the light could pass through the neighbouring gap on its return journey. He calculated the speed of light as the same as the wheels speed and showed Roemer's figure to be too low. However, in 1850, Leon Foucault devised an experiment in which a beam of light passed through a graduated scale and struck a spinning mirror. The spinning mirror in turn reflected the light onto a series of stationary mirrors. By the time the light had completed the journey; the spinning mirror had turned slightly and reflected the beam back along its original course with a tiny shift. This shift was measured and gave a figure of 298,000 km/s. We can now measure the speed of light accurately at 300,000 km per second when traveling in air.
Light travels at different speeds through different mediums. It slows down in water to 225,000 km/s and to 200,000 km/s in glass. Foucault showed that the speed of light is related to the refractive index of a substance. The more a substance bends light, the slower it travels.