| Themes > Science > Physics > About Physics, Generalities > A Brief History and Philosophy of Physics > The Development of Classical Physics: Mechanics, Heat, Optics, Electromagnetism, Atoms > Light and Optics |
The Greeks had applied the methods of geometry to the study of optics, and Ptolemy had a crude approximation to the law of refraction. This work was extended by the Arab Al-Hazen (965-1038), who showed that Ptolemy's law was just an approximation, valid at small angles. Al-Hazen also carried out experiments which brought him close to the thin lens formula for convex lenses. The telescope and compound microscope were invented in Holland near the beginning of the seventeenth century, with the telescope used to advantage by the early astronomers including Galileo. In 1621 Willebrod Snell rediscovered the correct formula for the refraction of light, which now bears his name. From the time of Descartes there was considerable debate as to whether light consisted of small particles which were localized and travelled in straight lines, or of waves which spread out in space. Descartes adhered to the former explanation whereas in the late 1600s Christian Huygens argued for a wave theory, with the waves travelling through an ether which permeated all space and all objects. Newton used a combination of the two approaches: while light itself consisted of "corpuscles", he believed that these particles could induce vibrations in the ether through which they travelled, which in turn could affect the transport of the particles. For example, he used this theory to explain "Newton's rings", alternating light and dark bands which appear when a slightly curved lens is placed in contact with a flat mirror. For a century after Newton, the majority of scientists adhered to the corpuscular theory. Thomas Young (1773-1829) revived the wave theory for light. It was generally accepted that sound was transported by waves carried through the air, and Young argued that light travelled in a similar way. He used the interference pattern produced in his famous "two-slit experiment", still studied in introductory physics courses today, as proof of this wave nature. (A similar pattern, in the form of a cross, can be seen with the naked eye by looking at a distant street light through a window screen, although using binoculars improves the image.) From these patterns he was able to measure the wavelength of light which he proved to be very small. He went on to show that this led to light travelling in approximately straight lines for the vast majority of common cases, although it did bend slightly around objects to produce patterns in their shadows, patterns which could be explained by his wave theory. Then, in 1817, the Frenchman Augustin Fresnel showed that all known optical phenomena could be explained by the wave theory provided that, following a suggestion of Young's, the vibrations were transverse (perpendicular to the direction of light propagation) rather than parallel to it as for sound waves. This firmly established the wave theory as dominant, although it did raise the question of how a fluid such as the ether could support a transverse vibration, since fluids usually have only longitudinal vibrations. This problem was a harbinger of an upcoming debate over the very existence of the ether. |
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