![]() Since the wavelength of visible light is less than a micron (a thousandth of a millimeter), it acts like a ray in the many common situations in which it encounters objects larger than a micron. Its wave characteristics are not pronounced in such situations. In the situations shown here, light interacts with objects large enough that it travels in straight lines, like a ray.Įxperiments show that when light interacts with an object several times larger than its wavelength, it travels in straight lines and acts like a ray. (c) Light can also reflect from an object like a mirror. (b) Light can reach a person by traveling through media like air and glass. (a) Light reaches the upper atmosphere of Earth, traveling through empty space directly from the source. Knowing the rotational speed of the wheel, the number of teeth on the wheel, and the distance to the mirror, Fizeau determined the speed of light to be 3.15 × 10 8 m/s, 3.15 × 10 8 m/s, which is only 5% too high.įigure 1.4 Three methods for light to travel from a source to another location. This could only happen if the wheel rotated through an angle corresponding to a displacement of ( n + ½ ) ( n + ½ ) teeth, while the pulses traveled down to the mirror and back. The speed of the wheel was then adjusted until no light returned to the observer located behind the wheel. An intense light source was placed behind the wheel, so that when the wheel rotated, it chopped the light beam into a succession of pulses. ![]() He placed a toothed wheel that could be rotated very rapidly on one hilltop and a mirror on a second hilltop 8 km away ( Figure 1.3). The first successful terrestrial measurement of the speed of light was made by Armand Fizeau (1819–1896) in 1849. Measurements of Io’s period done with the configurations of parts (a) and (b) differ, because the light path length and associated travel time increase from A to B (a) but decrease from A ′ A ′ to B ′ B ′ (b). By measuring the difference in these time intervals and with appropriate knowledge of the distance between Jupiter and Earth, Roemer calculated that the speed of light was 2.0 × 10 8 m/s, 2.0 × 10 8 m/s, which is 33% below the value accepted today.įigure 1.2 Roemer’s astronomical method for determining the speed of light. This time interval between the successive eclipses of Io seen at A ′ A ′ and B ′ B ′ is therefore less than the time interval between the eclipses seen at A and B. Since B ′ B ′ is closer to Jupiter than A ′ A ′, light takes less time to reach Earth when it is at B ′ B ′. The next eclipse then occurs when Earth is at point B ′ B ′, to which the light carrying the information of this eclipse must travel. The measurement of Io’s period begins with Earth at point A ′ A ′ and Io eclipsed by Jupiter. Now imagine it is about 6 months later, and the planets are arranged as in part (b) of the figure. Since B is farther from Jupiter than A, light takes more time to reach Earth when Earth is at B. The next time this alignment occurs, Earth is at point B, and the light carrying that information to Earth must travel to that point. When Earth is at point A, Earth, Jupiter, and Io are aligned. Figure 1.2(a) shows the planetary configurations when such a measurement is made from Earth in the part of its orbit where it is receding from Jupiter. Roemer found the period of revolution of Io by measuring the time interval between successive eclipses by Jupiter. ![]() Roemer realized that this fluctuation was due to the finite speed of light and could be used to determine c. He also discovered that this value fluctuated by a few seconds, depending on the position of Earth in its orbit around the Sun. ![]() He studied the orbit of Io, one of the four large moons of Jupiter, and found that it had a period of revolution of 42.5 h around Jupiter. The first measurement of the speed of light was made by the Danish astronomer Ole Roemer (1644–1710) in 1675. These facts have far-reaching implications, as we will see in later chapters. 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 improved, it was found that different observers, even those moving at large velocities with respect to each other, measure the same value for the speed of light. As you will see when you reach Relativity, it is a central concept in Einstein’s theory of relativity. The speed of light in a vacuum c is one of the fundamental constants of physics. ![]() List the ways in which light travels from a source to another location.Determine the index of refraction, given the speed of light in a medium.By the end of this section, you will be able to: ![]()
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