![]() ![]() The returning ray will be reflected into the observer’s eye only if the next mirror has rotated into the correct position just as the ray returns. As the mirrors rotate, the reflected ray is only briefly directed at the stationary mirror. A schematic of early apparatus used by Michelson and others to determine the speed of light. The time for the light to travel can be determined by how fast the mirrors must rotate for the light to be returned to the observer’s eye. Light reflected from a rotating set of mirrors was reflected from a stationary mirror 35 km away and returned to the rotating mirrors. One particularly direct method, used in 1887 by the American physicist Albert Michelson (1852–1931), is illustrated in Figure 2. In more recent times, physicists have measured the speed of light in numerous ways and with increasing accuracy. From his 1676 data, a value of the speed of light was calculated to be 2.26 x 10 8 m/s (only 25% different from today’s accepted value). He correctly concluded that the apparent change in period was due to the change in distance between Earth and Jupiter and the time it took light to travel this distance. Roemer had noted that the average orbital period of one of Jupiter’s moons, as measured from Earth, varied depending on whether Earth was moving toward or away from Jupiter. The first real evidence that light traveled at a finite speed came from the Danish astronomer Ole Roemer in the late 17th century. The Speed of LightĮarly attempts to measure the speed of light, such as those made by Galileo, determined that light moved extremely fast, perhaps instantaneously. So before we study the law of refraction, it is useful to discuss the speed of light and how it varies in different media. Why does light change direction when passing from one material (medium) to another? It is because light changes speed when going from one material to another. This bending of light is called refraction and is responsible for many optical phenomena. In this case, the light can reach the observer by two different paths, and so the fish seems to be in two different places. Looking at the fish tank as shown, we can see the same fish in two different locations, because light changes directions when it passes from water to air. The speed of light is so important that its value in a vacuum is one of the most fundamental constants in nature as well as being one of the four fundamental SI units. It makes connections between space and time and alters our expectations that all observers measure the same time for the same event, for example. These facts have far-reaching implications, as we will see in Chapter on Special Relativity. However, the speed of light does vary in a precise manner with the material it traverses. As the accuracy of the measurements of the speed of light were improved, c was found not to depend on the velocity of the source or the observer. The speed of light c not only affects refraction, it is one of the central concepts of Einstein’s theory of relativity.
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