Text 2. The speed of light

Key terms: speed of light, measured distance, single mirror, multisided mirror, stationary mirror, revolting mirror, telescope, light source, rotational frequency

The first recorded attempt to measure the speed of light took place in the seventeenth century. Galileo tried to measure the time it took for light to travel back and forth between two hilltops about 2 km apart. He stood on one hill at night with a covered lantern while his assistant stood on the other. They attempted to measure the time it took to travel the round trip distance of 4 km, but were unable to do so. Although Galileo increased the distance in subsequent trials, he could not obtain a time measurement. He concluded that either the speed of light is infinite, or it is so great that human reaction does not permit a measurement over such a short distance.

In 1675 Olaus Roemer, a Danish astronomer, attempted to measure the speed of light by observing the eclipse times for Jupiter’s largest moons. This astronomical method gave a value for the speed of light of 2.27x108 m/s.

Armand Fizeau [fizo:], a French physicist, was the first to successfully measure the speed of light by a terrestrial method in 1848. He used a rotating, toothed disc to interrupt the incident light. The disc was placed on one hilltop, and a mirror was placed on another hilltop about 3 km away. Light passed through a gap between two teeth, travelling to the mirror, and was reflected back. If the disk turned fast enough, the returning light passed through the next gap in the disc. From the rotational frequency of the disc and number of gaps around its circumference, he was able to measure the time it took the light to travel 6 km. Fizeau measured the speed of light to be 3.15 x 108 m/s ~ about 5% higher than we now believe it to be, Jean

Foucault [fuko:], another French physicist, replaced the rotating disc with a rotating mirror and obtained a more accurate value.

Albert Michelson, an American scientist, performed Foucalt’s method of measuring the time interval for light to travel a measured distance on Earth. He replaced Foucalt’s single mirror with a multi-sided mirror. Figure 7 shows the apparatus Michelson used.

Octagonal

fig.’7. The apparatus for measuring the speed of light

It consists of an intense light source, an eight-sided rotating merrier, a stationary mirror, and a telescope. The revolting mirror and stationary mirrors were placed on top of two mountains, 35 km apart from each other. Light was directed at one face of the revolting mirror. The light reflected from this face travelled the 70 kvi round trip to the stationary mirror and back. The time for the mirror to rotate through one eight of a revolution was calculated from the rotation period of the mirror. Then the speed of light in air was determined using this time and the 70 km distance. It was found to be 2.99789 x 108 m/s.

Similar experiments were done using an evacuated chamber to measure the speed of light in a vacuum For most calculations, whether for a vacuum or for air, the rounded value for the speed of light с is used. Thus с = 3.00 x 108 m/s. The speed of light in other materials is significantly less.