History Weblecture for Unit 49
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We have already seen how the ancient world and the Arab philosophers viewed light as a kind of ray, perhaps sent out by the eye, which followed a predictable path reflecting from shiny objects and refracting through transparent ones, before returning to the eye. The Renaissance scientists René Descartes and Isaac Newton also wrote about light, but they both believed it to be made up of corpuscles, or tiny particles, traveling at high speed through the "aether".
In the Optics, Isaac Newton also speculated that light might be a vibration of the aether. Other philosophers, like Christiaan Huygens, proposed a wave propagation method for light. The main problem with this theory, prior to about 1800, was that no one had observed diffraction in light. When sound and water waves encounter an obstacle, they bend around the obstacle. Light, however, did not appear to bend; instead, it cast sharp shadows.
In the late 18th century, the Danish astronomer Ole Roemer used Kepler's laws to predict the positions of the moons of Jupiter, then timed them with the newly invented pendulum clock. He showed that the moons' periods were about 26 seconds longer when the earth was moving away from Jupiter than when it was moving toward Jupiter, and that this difference was due to the difference in the position of the earth with respect to Jupiter. Christian Huygen used Roemer's information to calculated the speed of light at approximated 3/4 of the modern value, which is 186,000 miles per second, or 300000 kilometers per second.
Take a look at this brief description of Roemer's method. [This is one of my very favorite websites -- explore the rest of it at your own risk!]
A Scottish scientist Thomas Young (a student of Joseph Black's), performed what is now called the double slit experiment. He was able to determine that light behaved like a wave in water passing a barrier edge.
Young presented his results to the Royal Society in 1801. He thought the waves were longitudinal or made up of alternately compressed and rarefied particles, which is how sound waves move through air. However, there was one characteristic of light that could not be explained by a longitudinal wave, and that was the ability to polarize light. It was well-known that if two pieces of calcite crystal (Iceland spar) were placed in front of a beam of light and rotated so that one was at a 90° angle to the other, the light would be blocked. If the crystals are aligned, the light passes through both apparently unchanged.
In order to explain this, we must assume that light is a transverse wave like the wave in a vibrating string. If the crystal acts like a slit, then light can pass through it if the slit is aligned with the up and down motion of the light wave. If the crystal is turned 90° to the wave, the slit is too narrow for the wave to pass through, and the light is blocked.
A major problem remained, however: if light was a wave, what was the medium which supported it? Sound travels by compressing particles of gas in air together; a transverse wave must move through water or down a string. Light, however, can move through an apparent vacuum. To support the theory of wave propagation for light, Young and his supporters like the Frenchman Augustin Fresnel proposed a luminiferous aether. Fresnel was a good mathematician, and worked out the calculations required to describe this kind of light wave. However, they could not show that such a thing actually existed.
While the debate over the nature of light continued, other investigators were busy studying the characteristics of the light spectrum. Newton had show that white light was actually composed of many different colors. In 1800, the astronomer William Herschel (who discovered the planet Uranus) placed a thermometer below the visible red portion of sunlight refracted through his prism. The temperature of the thermometer increased, and he concluded that an "invisible" light was transmitting heat from the sun to the thermometer. We identify this now as infrared light.
The German physicist Joseph von Fraunhofer looked at sunlight more closely, and observed that the apparently continuous spectrum of light coming from the sun actually had some black marks in it. In some cases, the position of black marks in the spectrum matched the pattern of bright single lines observed by Robert Bunsen and and Gustav Kirchhoff when they burned metals in different gases. Bunsen identified a number of elements and compounds by their emission spectra.
Examine the solar spectrum between 4300 and 4400 Angstroms (the band should be a continues line but is "wrapped" so that it will fit the webpage). The very thick line at about 4320 is the Balmer H-gamma line. It corresponds to the exact amount of energy required by a hydrogen electron to jump from a low second-level orbital to a higher level 5 orbital.
Now take a look at the actual equipment used by Kirchhoff and Bunsen, as reported in their publication of 1860.
Kirchhoff showed that a cool gas would actually absorbe light selectively, blocking out some colors from a hot source while letting others through. The use of spectroscopy, or the identification of spectrum lines in absorption spectra makes it possible to identify gases in the atmospheres of stars. Its use in the investigation of emission spectra or light emitted by a glowing body makes it possible to identify the chemicals which are giving off the light.
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