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Newton and Mechanics

phase waves, Dutch astronomer, luminiferous ether, longitudinal waves, light wave

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The apparent linear propagation of light was known since antiquity, and the ancient Greeks believed that light consisted of a stream of corpuscles. They were, however, quite confused as to whether these corpuscles originated in the eye or in the object viewed. Any satisfactory theory of light must explain its origin and disappearance and its changes in speed and direction while it passes through various media. Partial answers to these questions were proposed in the 17th century by Newton, who based them on the assumptions of a corpuscular theory, and by the English scientist Robert Hooke and the Dutch astronomer, mathematician, and physicist Christiaan Huygens, who proposed a wave theory. No experiment could be performed that distinguished between the two theories until the demonstration of interference in the early 19th century by the British physicist and physician Thomas Young. The French physicist Augustin Jean Fresnel decisively favored the wave theory.

Interference can be demonstrated by placing a thin slit in front of a light source, stationing a double slit farther away, and looking at a screen spaced some distance behind the double slit. Instead of showing a uniformly illuminated image of the slits, the screen will show equispaced light and dark bands. Particles coming from the same source and arriving at the screen via the two slits could not produce different light intensities at different points and could certainly not cancel each other to yield dark spots. Light waves, however, can produce such an effect. Assuming, as did Huygens, that each of the double slits acts as a new source, emitting light in all directions, the two wave trains arriving at the screen at the same point will not generally arrive in phase, though they will have left the two slits in phase. Depending on the difference in their paths, “positive” displacements arriving at the same time as “negative” displacements of the other will tend to cancel out and produce darkness, while the simultaneous arrival of either positive or negative displacements from both sources will lead to reinforcement or brightness. Each apparent bright spot undergoes a timewise variation as successive in-phase waves go from maximum positive through zero to maximum negative displacement and back. Neither the eye nor any classical instrument, however, can determine this rapid “flicker,” which in the visible-light range has a frequency from 4 ? 1014 to 7.5 ? 1014 Hz, or cycles per second. Although it cannot be measured directly, the frequency can be inferred from wavelength and velocity measurements. The wavelength can be determined from a simple measurement of the distance between the two slits, and the distance between adjacent bright bands on the screen; it ranges from 4 ? 10-5 cm (1.6 ? 10-5 in) for violet light to 7.5 ? 10-5 cm (3 ? 10-5 in) for red light with intermediate wavelengths for the other colors.

The first measurement of the velocity of light was carried out by the Danish astronomer Olaus Roemer in 1676. He noted an apparent time variation between successive eclipses of Jupiter's moons, which he ascribed to the intervening change in the distance between Earth and Jupiter, and to the corresponding difference in the time required for the light to reach the earth. His measurement was in fair agreement with the improved 19th-century observations of the French physicist Armand Hippolyte Louis Fizeau, and with the work of the American physicist Albert Abraham Michelson and his coworkers, which extended into the 20th century. Today the velocity of light is known very accurately as 299,292.6 km (185,971.8 mi sec) in vacuum. In matter, the velocity is less and varies with frequency, giving rise to a phenomenon known as dispersion. See also Optics; Spectrum; Vacuum.

Maxwell's work contributed several important results to the understanding of light by showing that it was electromagnetic in origin and that electric and magnetic fields oscillated in a light wave. His work predicted the existence of nonvisible light, and today electromagnetic waves or radiations are known to cover the spectrum from gamma rays (see Radioactivity), with wavelengths of 10-12 cm (4 ? 10-11 in), through X rays, visible light, microwaves, and radio waves, to long waves of hundreds of kilometers in length (see X Ray). It also related the velocity of light in vacuum and through media to other observed properties of space and matter on which electrical and magnetic effects depend. Maxwell's discoveries, however, did not provide any insight into the mysterious medium, corresponding to the string, through which light and electromagnetic waves had to travel (see the Electricity and Magnetism section above). Based on the experience with water, sound, and elastic waves, scientists assumed a similar medium to exist, a “luminiferous ether” without mass, which was all-pervasive (because light could obviously travel through a massless vacuum), and had to act like a solid (because electromagnetic waves were known to be transverse and the oscillations took place in a plane perpendicular to the direction of propagation, and gases and liquids could only sustain longitudinal waves, such as sound waves). The search for this mysterious ether occupied physicists' attention for much of the last part of the 19th century.

The problem was further compounded by an extension of a simple problem. A person walking forward with a speed of 3.2 km/h (2 mph) in a train traveling at 64.4 km/h (40 mph) appears to move at 67.6 km/h (42 mph), to an observer on the ground. In terms of the velocity of light the question that now arose was: If light travels at about 300,000 km/sec (about 186,000 mi/sec) through the ether, at what velocity should it travel relative to an observer on earth while the earth also moves through the ether? Or, alternately, what is the earth's velocity through the ether? The famous Michelson-Morley experiment, first performed in 1887 by Michelson and the American chemist Edward Williams Morley using an interferometer, was an attempt to measure this velocity; if the earth were traveling through a stationary ether, a difference should be apparent in the time taken by light to traverse a given distance, depending on whether it travels in the direction of or perpendicular to the earth's motion. The experiment was sensitive enough to detect even a very slight difference by interference; the results were negative. Physics was now in a profound quandary from which it was not rescued until Einstein formulated his theory of relativity in 1905.

The Development of Mechanics

A branch of physics that assumed major stature during the 19th century was thermodynamics. It began by disentangling the previously confused concepts of heat and temperature, by arriving at meaningful definitions, and by showing how they could be related to the heretofore purely mechanical concepts of work and energy.

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