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This topic is reminiscent of my study Geology at the Rijks Universiteit Utrecht, major Geochemistry - well, actually Mineralogy, which was not a possible major at that time.

The Nature of Light

Light is radiant energy of a wavelength that can stimulate our visual sense. As with electricity and other forms of energy, we know it by its effects, and can predict its behavior and use it without fully understanding its nature.

The early Greeks considered light to be a stream of minute particles, called corpuscles, which either were emitted by the eye or entered the eye from a luminous body. This theory remained virtually unchallenged until the late seventeenth century, when the Dutch scientist Christiaan Huygens (1629-1695), in attempting to explain diffraction, refraction, interference, polarization, and other optical phenomena, proposed that light is propagated as longitudinal waves. But Huygens' contemporary, Sir Isaac Newton (1642-1727), favored the older corpuscular theory, and because of his stature as a scientist the wave concept was largely disregarded until the early nineteenth century. The work of Thomas Young (1773-1829), Augustin J. Fresnel (1788-1827). Jean B.L. Foucault (1819-1868), and others eventually led to the acceptance ot the theory that light is a wave phenomenon - but that the waves are transverse rather than longitudinal as Huygens had suggested.

The wave theory of light, either transverse or longitudinal, was still not without major objections, however, for even its foremost defenders were not completely satisfied with its adequacy. Perhaps for them the major problem was that waves require a transmitting medium for their propagation; accordingly, the supporters of the wave theory postulated the existence of an ether - a special medium that fills all space and is endowed with some highly improbable physical properties.

The independent discovery by Michael Faraday (1791-1867) and Joseph Henry (1797-1878) of electrical induction and the subsequent discovery by Faraday of the first magneto-optic phenomenon were instrumental in drawing the attention of James Clerk Maxwell (1831-1879) to the possible connection between light, electricity, and magnetism. Through his electromagnetic theory Maxwell showed that electricity and magnetism were inseparable phenomena. He concluded that if his theory were correct, electromagnetic waves must travel at the velocity of light, and, furthermore, that light itself must be an electromagnetic phenomenon. It was Heinrich Hertz (1857-1894) who provided experimental confirmation of Maxwell's theory by demonstrating that electromagnetic disturbances have measurable wavelengths and generally possess the properties of light waves.

For a while it seemed that Hertz's work represented the last chapter to be written on the nature of light, but with the beginning of the twentieth century came the dawn of a new era in the physical sciences - an era in which the work of such intellectual giants as J.J. Thomson (1856-1940), Ernest Rutherford (1871-1937), Niels Bohr (1885-1962), Henry Moseley (1887-1915), Max Planck (1858-1947), and Albert Einstein (1879-1955) gradually revealed the inner world of the atom. Besides being the first to propose a theory of atomic structure, Thomson also introduced the notion that electricity is of a corpuscular nature. For several decades the world of physics was divided over whether various kinds of radiation were wave phenomena or corpuscular phenomena. Planck's work on black-body radiation led him to postulate, in 1900, that a light source emits its radiation in discrete quanta, or particles, rather than continuously as the electromagnetic theory held. The quantum theory was so revolutionary, however, that it was generally disregarded until Albert Einstein made use of it in 1905 to explain the photoelectric effect - a phenomenon that Hertz had discovered before the turn of the century but which had never been explained in terms of the electromagnetic theory.

Physicists must still resort to using two seemingly contradictionary theories to explain various luminous phenomena. Although the quantum theory and wave mechanics have supplied explanations for certain phenomena that could not be explained in terms of the electromagnetic theory, they did not lead to the development of a unified theory of light. Some phenomena can be explained only in terms of the wave theory, and others only in terms of the quantum concept. The two theories may soon be reconciled, however, for the equivalence of matter and energy has been demonstrated repeatedly. Moreover, not only do light quanta (photons) behave like particles of matter, but electrons behave like groups of waves.

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