LIGHT

electromagnetic radiation that can be detected by the human eye. In terms of wavelength, electromagnetic radiation occurs over an extremely wide range, from gamma rays with a wavelength of 3 10-14 centimetre to long radio waves measured in millions of kilometres. In that spectrum the wavelengths visible to humans occupy a very narrow band, from about 7 10-5 centimetre (red light) down to about 4 10-5 centimetre (violet). The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299,792,458 metres per second, or about 186,282 miles per second (299,792 kilometres per second). Light, a basic aspect of the human environment, cannot be defined in terms of anything simpler or more directly appreciated by the senses than itself. Light, certainly, is responsible for the sensation of sight. It is propagated with a speed that is high but not infinitely high. Physicists are acquainted with two methods of propagation from one place to another, as (1) particles and as (2) waves, and for a long time they have sought to define light in terms of either particles or waves. In the early 19th century a wave description was favoured, though it was difficult to understand what kind of wave could possibly be propagated across the near-vacuum of interstellar space and with the extremely high speed of almost 300,000 kilometres per second. In the latter half of the 19th century a British physicist, James Clerk Maxwell, showed that certain electromagnetic effects could be propagated through a vacuum with a speed equal to the measured speed of light. Thus, in the second half of the 19th century, light was described as electromagnetic waves. Such waves were visualized as analogous to those on the surface of water (transverse waves) but with an extremely short wavelength of about 500 nanometres (one nanometre is 10 -9 metre). The analogy is valid up to a certain point but the experimental results obtained at the end of the 19th century and in the early years of the 20th century revealed properties of light that could not have been predicted from knowledge that was obtainable about other waves. These results led to the quantum theory of light, which in its primitive form asserted that, at least in regard to its emission and absorption by matter, light behaves like particles rather than waves. The results of certain important experiments on the spreading of light into shadows and other experiments (on the interaction of beams of light) that supported the wave theory found no place in a particle theory. For a time it was believed that light could not be adequately described by analogy with either waves or particlesthat it could be defined only by a description of its properties. A reconciliation of wave and particle concepts did not emerge until after 1924. Two properties of light are, perhaps, more basic and fundamental than any others. The first of these is that light is a form of energy conveyed through empty space at high velocity (in contrast, many forms of energy, such as the chemical energy stored in coal or oil, can be transferred from one place to another only by transporting the matter in which the energy is stored). The unique property of light is, thus, that energy in the form of light is always moving, and its movement is only in an indirect way affected by motion of the matter through which it is moving. (When light energy ceases to move, because it has been absorbed by matter, it is no longer light.) The second fundamental property is that a beam of light can convey information from one place to another. This information concerns both the source of light and also any objects that have partly absorbed or reflected or refracted the light before it reaches the observer. More information reaches the human brain through the eyes than through any other sense organ. Even so, the visual system extracts only a minute fraction of the information that is imprinted on the light that enters the eye. Optical instruments extract much more information from the visual scene; spectroscopic instruments, for example, reveal far more about a source of light than the eye can discover by noting its colour, and telescopes and microscopes extract scientific information from the environment. Modern optical instruments produce, indeed, so much information that automatic methods of recording and analysis are needed to enable the brain to comprehend it. From the standpoint of wave motion, blue light has a somewhat higher frequency and shorter wavelength than red. In the quantum theory, blue light consists of higher energy quanta than the red. The subject of light is so wide and its associations are so numerous that it cannot be accommodated within one article of reasonable length. There are three main divisions of the subject of light: physical optics, physiological optics, and optical instrumentation. This article deals primarily with physical optics, treating the nature and behaviour of light. It also discusses the interaction of light with matter. Although electromagnetic theory is considered here, further elucidation may be obtained in the article electromagnetic radiation. The article photoreception includes the physiological and psychological aspects of light, while the article optics treats the practical application of light. The experimental evidence that led to the quantum theory of radiation is included in the present article along with a brief statement of some of the basic ideas. The quantum theory of radiation, however, is so closely associated with the quantum theory of matter that the two must be considered together, as is done in the article quantum mechanics. Robert William Ditchburn The Editors of the Encyclopdia Britannica electromagnetic radiation that can be detected by the human eye. In terms of wavelength, electromagnetic radiation occurs over an extremely wide range, from gamma rays with a wavelength of 3 10-14 centimetre to long radio waves measured in millions of kilometres. In that spectrum the wavelengths visible to humans occupy a very narrow band, from about 7 10-5 centimetre (red light) down to about 4 10-5 centimetre (violet). The spectral regions adjacent to the visible band are often referred to as light also, infrared at the one end and ultraviolet at the other. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299,792,458 metres per second, or about 186,282 miles per second (299,792 kilometres per second). Light is treated in a number of articles. For the basic properties of light, see light; optics. For more general electromagnetic phenomena, see electromagnetic radiation; radiation. For the dual nature of light (as wave and as particle) and the current quantum mechanical understanding of optical phenomena, see mechanics. For the fundamental role of light in relativity theory, see relativity. For the role of light in physiological processes, see photoreception; perception; photosynthesis. For optical instrumentation and methods of analysis, see spectroscopy; microscope; telescope. For practical and engineering applications, see photography. Additional reading General works A.C.S. van Heel and C.H.F. Velzel, What Is Light? (1968; originally published in Dutch, 1968), is a good book for the general reader; numerous illustrations are accompanied by a simple text. Marcel Minnaert, Light and Colour in the Outdoors (1993; originally published in Dutch, 1937), also recommended for the general reader, deals with optical effects such as rainbows, mirages, and many other effects of light and colour. Max Born and Emil Wolf, Principles of Optics, 6th ed. (1980, reprinted 1993), is a comprehensive treatise on the wave theory of light, particularly strong on diffraction, scattering, polarization, and coherencesuitable for the reader with advanced mathematical knowledge. R.W. Ditchburn, Light (1953, reprinted 1991), is a text at the university level that includes both theory and experimental evidence in relation to wave phenomena and quantum optics. K.D. Froome and L. Essen, The Velocity of Light and Radio Waves (1969), critically reviews modern methods of measuring the constant c. The work by J.H. Sanders, Velocity of Light (1965), contains reprints of original papers by A.A. Michelson, F.G. Pease, F. Pearson, Louis Essen, A.C. Gordon-Smith, and E. Bergstrand, together with a review of other work on the subject. The history of early theories is traced in A.I. Sabra, Theories of Light, From Descartes to Newton (1967, reissued 1981). Richard Morris, Light (1979), provides a general introduction with good bibliographies. Michael I. Sobel, Light (1987), is an interesting, understandable, yet technical account with historical and practical examples. Quantum mechanics and electromagnetic theory Leonard I. Schiff, Quantum Mechanics, 3rd ed. (1968), is a systematic exposition of the physical concepts of quantum mechanics, including a description of the electromagnetic field and its interactions with matter. Joseph Needham, Science and Civilisation in China, vol. 4, pt. 1 (1962), includes an authoritative account of the study of light in ancient China and a detailed comparison with parallel developments due to Greek and Arab philosophers. Edmund Whittaker, A History of the Theories of Aether & Electricity, rev. and enlarged ed., 2 vol. (195153, reissued in 1 vol., 1989), deals with the background to the development of the electromagnetic theory of light in the 19th century. F.H. Read, Electromagnetic Radiation (1980), includes lucid explanations of the basic concepts. Robert William Ditchburn The Editors of the Encyclopdia Britannica