in terms of classical theory, the flow of energy at the universal speed of light through free space or through a material medium in the form of the electric and magnetic fields that make up electromagnetic waves such as radio waves, visible light, and gamma rays. In such a wave, time-varying electric and magnetic fields are mutually linked with each other at right angles and perpendicular to the direction of motion. An electromagnetic wave is characterized by its intensity and the frequency n of the time variation of the electric and magnetic fields. In terms of the modern quantum theory, electromagnetic radiation is the flow of photons (also called light quanta) through space. Photons are packets of energy hn that always move with the universal speed of light. The symbol h is Planck's constant, while the value of n is the same as that of the frequency of the electromagnetic wave of classical theory. Photons having the same energy hn are all alike, and their number density corresponds to the intensity of the radiation. Electromagnetic radiation exhibits a multitude of phenomena as it interacts with charged particles in atoms, molecules, and larger objects of matter. These phenomena as well as the ways in which electromagnetic radiation is created and observed, the manner in which such radiation occurs in nature, and its technological uses depend on its frequency n. The spectrum of frequencies of electromagnetic radiation extends from very low values over the range of radio waves, television waves, and microwaves to visible light and beyond to the substantially higher values of ultraviolet light, X rays, and gamma rays. The basic properties and behaviour of electromagnetic radiation are discussed in this article, as are its various forms, including their sources, distinguishing characteristics, and practical applications. The article also traces the development of both the classical and quantum theories of radiation. energy that is propagated through free space or through a material medium in the form of electromagnetic waves, such as radio waves, visible light, and gamma rays. The term also refers to the emission and transmission of such radiant energy. The Scottish physicist James Clerk Maxwell was the first to predict the existence of electromagnetic waves. In 1864 he set forth his electromagnetic theory, proposing that lightincluding various other forms of radiant energyis an electromagnetic disturbance in the form of waves. In 1887 Heinrich Hertz, a German physicist, provided experimental confirmation by producing the first man-made electromagnetic waves and investigating their properties. Subsequent studies resulted in a broader understanding of the nature and origin of radiant energy. It has been established that time-varying electric fields can induce magnetic fields and that time-varying magnetic fields can in like manner induce electric fields. Because such electric and magnetic fields generate each other, they occur jointly, and together they propagate as electromagnetic waves. An electromagnetic wave is a transverse wave in that the electric field and the magnetic field at any point and time in the wave are perpendicular to each other as well as to the direction of propagation. In free space (i.e., a space that is absolutely devoid of matter and that experiences no intrusion from other fields or forces), electromagnetic waves always propagate with the same speedthat of light (299,792,458 m per second, or 186,282 miles per second)independent of the speed of the observer or of the source of the waves. Electromagnetic radiation has properties in common with other forms of waves such as reflection, refraction, diffraction, and interference. Moreover, it may be characterized by the frequency with which it varies over time or by its wavelength. Electromagnetic radiation, however, has particle-like properties in addition to those associated with wave motion. It is quantized in that for a given frequency n, its energy occurs as an integer times hn, in which h is a fundamental constant of nature known as Planck's constant. A quantum of electromagnetic energy is called a photon. Visible light and other forms of electromagnetic radiation may be thought of as a stream of photons, with photon energy directly proportional to frequency. The electromagnetic spectrum Electromagnetic radiation spans an enormous range of frequencies or wavelengths, as is shown by the electromagnetic spectrum (see Figure). Customarily, it is designated by fields, waves, and particles in increasing magnitude of frequenciesradio waves, microwaves, infrared rays, visible light, ultraviolet light, X rays, and gamma rays. The corresponding wavelengths are inversely proportional, and both the frequency and wavelength scales are logarithmic. Electromagnetic radiation of different frequencies interacts with matter differently. A vacuum is the only perfectly transparent medium, and all material media absorb strongly some regions of the electromagnetic spectrum. For example, molecular oxygen (O2), ozone (O3), and molecular nitrogen (N2) in the Earth's atmosphere are almost perfectly transparent to infrared rays of all frequencies, but they strongly absorb ultraviolet light, X rays, and gamma rays. The frequency (or energy equal to hv) of X rays is substantially higher than that of visible light, and so X rays are able to penetrate many materials that do not transmit light. Moreover, absorption of X rays by a molecular system can cause chemical reactions to occur. When X rays are absorbed in a gas, for instance, they eject photoelectrons from the gas, which in turn ionize its molecules. If these processes occur in living tissue, the photoelectrons emitted from the organic molecules destroy the cells of the tissue. Gamma rays, though generally of somewhat higher frequency than X rays, have basically the same nature. When the energy of gamma rays is absorbed in matter, its effect is virtually indistinguishable from the effect produced by X rays. There are many sources of electromagnetic radiation, both natural and man-made. Radio waves, for example, are produced by cosmic objects such as pulsars and quasars and by electronic circuits. Sources of ultraviolet radiation include mercury vapour lamps and high-intensity lights, as well as the Sun. The latter also generates X rays, as do certain types of particle accelerators and electronic devices. Additional reading Accounts of the historical development of electromagnetic theories may be found in Isaac Asimov, The History of Physics (1984); I. Bernard Cohen, Revolution in Science (1985); and Thomas S. Kuhn, Black-Body Theory and the Quantum Discontinuity, 18941912 (1978, reprinted 1987). Early works include Edmund Whittaker, A History of the Theories of Aether and Electricity, rev. and enlarged ed., 2 vol. (195153); and Heinrich Hertz, Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space (1893, reissued 1962; originally published in German, 1892). Ivan Tolstoy, James Clerk Maxwell (1981), recounts the life of this pivotal figure, as well as his theory and its ramifications. James Clerk Maxwell: A Commemoration Volume, 18311931 (1931), includes essays by Max Planck and Albert Einstein, among others. Extensive treatments of visible radiation (light) are given by Michael I. Sobel, Light (1987); Max Born and Emil Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference, and Diffraction of Light, 6th ed. (1987); and Francis A. Jenkins and Harvey E. White, Fundamentals of Optics, 4th ed. (1976). Classical radiation and electron theory are treated in John David Jackson, Classical Electrodynamics, 2nd ed. (1975); and Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, 3 vol. (196365; vol. 1 and 2 have been reprinted, 1977). Waveparticle dualism is addressed by Louis De Broglie, Matter and Light (1939, reissued 1955; originally published in French, 1937); S. Diner et al. (eds.), The WaveParticle Dualism (1984); and A.B. Arons, The Development of Concepts of Physics: From the Rationalization of Mechanics to the First Theory of Atomic Structure (1965). Quantum electrodynamics is discussed in Richard P. Feynman, QED: The Strange Theory of Light and Matter (1985); Rodney Loudon, The Quantum Theory of Light, 2nd ed. (1983); W. Heitler, The Quantum Theory of Radiation, 3rd ed. (1964, reprinted 1984); J.M. Jauch and F. Rohrlich, The Theory of Photons and Electrons: The Relativistic Quantum Field Theory of Charged Particles with Spin One-half, 2nd expanded ed. (1976); and Paul Davies (ed.), The New Physics (1989). Hellmut Fritzsche Forms of electromagnetic radiation Electromagnetic radiation appears in a wide variety of forms and manifestations. Yet, these diverse phenomena are understood to comprise a single aspect of nature, following simple physical principles. Common to all forms is the fact that electromagnetic radiation interacts with and is generated by electric charges. The apparent differences in the phenomena arise from the question in which environment and under what circumstances can charges respond on the time scale of the frequency n of the radiation. At smaller frequencies n (smaller than 1012 hertz), electric charges typically are the freely moving electrons in the metal components of antennas or the free electrons and ions in space that give rise to phenomena related to radio waves, radar waves, and microwaves. At higher frequencies (1012 to 5 1014 hertz), in the infrared region of the spectrum, the moving charges are primarily associated with the rotations and vibrations of molecules and the motions of atoms bonded together in materials. Electromagnetic radiation in the visible range to X rays have frequencies that correspond to charges within atoms, whereas gamma rays are associated with frequencies of charges within atomic nuclei. The characteristics of electromagnetic radiation occurring in the different regions of the spectrum are described in this section. Radio waves Radio waves are used for wireless transmission of sound messages, or information, for communication, as well as for maritime and aircraft navigation. The information is imposed on the electromagnetic carrier wave as amplitude modulation (AM) or as frequency modulation (FM) or in digital form (pulse modulation). Transmission therefore involves not a single-frequency electromagnetic wave but rather a frequency band whose width is proportional to the information density. The width is about 10,000 Hz for telephone, 20,000 Hz for high-fidelity sound, and five megahertz (MHz = one million hertz) for high-definition television. This width and the decrease in efficiency of generating electromagnetic waves with decreasing frequency sets a lower frequency limit for radio waves near 10,000 Hz. Because electromagnetic radiation travels in free space in straight lines, scientists questioned the efforts of the Italian physicist and inventor Guglielmo Marconi to develop long-range radio. The curvature of the Earth limits the line-of-sight distance from the top of a 100-metre (330-foot) tower to about 30 kilometres (19 miles). Marconi's unexpected success in transmitting messages over more than 2,000 kilometres led to the discovery of the KennellyHeaviside layer, more commonly known as the ionosphere. This region is an approximately 300-kilometre-thick layer starting about 100 kilometres above the Earth's surface in which the atmosphere is partially ionized by ultraviolet light from the Sun, giving rise to enough electrons and ions to affect radio waves. Because of the Sun's involvement, the height, width, and degree of ionization of the stratified ionosphere vary from day to night and from summer to winter. Figure 5: Radio-wave transmission reaching beyond line of sight by means of the sky wave reflected Radio waves transmitted by antennas in certain directions are bent or even reflected back to Earth by the ionosphere, as illustrated in Figure 5. They may bounce off the Earth and be reflected by the ionosphere repeatedly, making radio transmission around the globe possible. Long-distance communication is further facilitated by the so-called ground wave. This form of electromagnetic wave closely follows the surface of the Earth, particularly over water, as a result of the wave's interaction with the terrestrial surface. The range of the ground wave (up to 1,600 kilometres) and the bending and reflection of the sky wave by the ionosphere depend on the frequency of the waves. Under normal ionospheric conditions 40 MHz is the highest-frequency radio wave that can be reflected from the ionosphere. In order to accommodate the large band width of transmitted signals, television frequencies are necessarily higher than 40 MHz. Television transmitters must therefore be placed on high towers or on hilltops. As a radio wave travels from the transmitting to the receiving antenna, it may be disturbed by reflections from buildings and other large obstacles. Disturbances arise when several such reflected parts of the wave reach the receiving antenna and interfere with the reception of the wave. Radio waves can penetrate nonconducting materials such as wood, bricks, and concrete fairly well. They cannot pass through electrical conductors such as water or metals. Above n = 40 MHz, radio waves from deep space can penetrate the Earth's atmosphere. This makes radio astronomy observations with ground-based telescopes possible. Whenever transmission of electromagnetic energy from one location to another is required with minimal energy loss and disturbance, the waves are confined to a limited region by means of wires, coaxial cables, and, in the microwave region, waveguides. Unguided or wireless transmission is naturally preferred when the locations of receivers are unspecified or too numerous, as in the case of radio and television communications. Cable television, as the name implies, is an exception. In this case electromagnetic radiation is transmitted by a coaxial cable system to users either from a community antenna or directly from broadcasting stations. The shielding of this guided transmission from disturbances provides high-quality signals. Figure 6: Cross section of a coaxial cable carrying high-frequency Figure 2: Radiation fields in which vectors and are perpendicular to each other Figure 6 shows the electric field E (solid lines) and the magnetic field B (dashed lines) of an electromagnetic wave guided by a coaxial cable. There is a potential difference between the inner and outer conductors and so electric field lines E extend from one conductor to the other, represented here in cross section. The conductors carry opposite currents that produce the magnetic field lines B. The electric and magnetic fields are perpendicular to each other and perpendicular to the direction of propagation, as is characteristic of the electromagnetic waves illustrated in Figure 2. At any cross section viewed, the directions of the E and B field lines change to their opposite with the frequency n of the radiation. This direction reversal of the fields does not change the direction of propagation along the conductors. The speed of propagation is again the universal speed of light if the region between the conductors consists of air or free space. A combination of radio waves and strong magnetic fields is used by magnetic resonance imaging (MRI) to produce diagnostic pictures of parts of the human body and brain without apparent harmful effects. This imaging technique has thus found increasingly wider application in medicine (see also radiation). Extremely low-frequency (ELF) waves are of interest for communications systems for submarines. The relatively weak absorption by seawater of electromagnetic radiation at low frequencies and the existence of prominent resonances of the natural cavity formed by the Earth and the ionosphere make the range between 5 and 100 Hz attractive for this application. There is evidence that ELF waves and the oscillating magnetic fields that occur near electric power transmission lines or electric heating blankets have adverse effects on human health and the electrochemical balance of the brain. Prolonged exposure to low-level and low-frequency magnetic fields have been reported to increase the risk of developing leukemia, lymphoma, and brain cancer in children. Historical survey Development of the classical radiation theory The classical electromagnetic radiation theory remains for all time one of the greatest triumphs of human intellectual endeavor. So said Max Planck in 1931, commemorating the 100th anniversary of the birth of the Scottish physicist James Clerk Maxwell, the prime originator of this theory. The theory was indeed of great significance, for it not only united the phenomena of electricity, magnetism, and light in a unified framework but also was a fundamental revision of the then-accepted Newtonian way of thinking about the forces in the physical universe. The development of the classical radiation theory constituted a conceptual revolution that lasted for nearly half a century. It began with the seminal work of the British physicist and chemist Michael Faraday, who published his article Thoughts on Ray Vibrations in Philosophical Magazine in May 1846, and came to fruition in 1888 when Hertz succeeded in generating electromagnetic waves at radio and microwave frequencies and measuring their properties. Wave theory and corpuscular theory The Newtonian view of the universe may be described as a mechanistic interpretation. All components of the universe, small or large, obey the laws of mechanics, and all phenomena are in the last analysis based on matter in motion. A conceptual difficulty in Newtonian mechanics, however, is the way in which the gravitational force between two massive objects acts over a distance across empty space. Newton did not address this question, but many of his contemporaries hypothesized that the gravitational force was mediated through an invisible and frictionless medium which Aristotle had called the ether (or aether). The problem is that everyday experience of natural phenomena shows mechanical things to be moved by forces which make contact. Any cause and effect without a discernable contact, or action at a distance, contradicts common sense and has been an unacceptable notion since antiquity. Whenever the nature of the transmission of certain actions and effects over a distance was not yet understood, the ether was resorted to as a conceptual solution of the transmitting medium. By necessity, any description of how the ether functioned remained vague, but its existence was required by common sense and thus not questioned. In Newton's day, light was one phenomenon, besides gravitation, whose effects were apparent at large distances from its source. Newton contributed greatly to the scientific knowledge of light. His experiments revealed that white light is a composite of many colours, which can be dispersed by a prism and reunited to again yield white light. The propagation of light along straight lines convinced him that it consists of tiny particles which emanate at high or infinite speed from the light source. The first observation from which a finite speed of light was deduced was made soon thereafter, in 1676, by the Danish astronomer Ole Rmer (see Speed of light below). Observations of two phenomena strongly suggested that light propagates as waves. One of these involved interference by thin films, which was discovered in England independently by Robert Boyle and Robert Hooke. The other had to do with the diffraction of light in the geometric shadow of an opaque screen. The latter was also discovered by Hooke, who published a wave theory of light in 1665 to explain it. The Dutch scientist Christiaan Huygens greatly improved the wave theory and explained reflection and refraction in terms of what is now called Huygens' principle. According to this principle (published in 1690), each point on a wave front in the hypothetical ether or in an optical medium is a source of a new spherical light wave and the wave front is the envelope of all the individual wavelets that originate from the old wave front. In 1669 another Danish scientist, Erasmus Bartholin, discovered the polarization of light by double refraction in Iceland spar (calcite). This finding had a profound effect on the conception of the nature of light. At that time, the only waves known were those of sound, which are longitudinal. It was inconceivable to both Newton and Huygens that light could consist of transverse waves in which vibrations are perpendicular to the direction of propagation. Huygens gave a satisfactory account of double refraction by proposing that the asymmetry of the structure of Iceland spar causes the secondary wavelets to be ellipsoidal instead of spherical in his wave front construction. Since Huygens believed in longitudinal waves, he failed, however, to understand the phenomena associated with polarized light. Newton, on the other hand, used these phenomena as the bases for an additional argument for his corpuscular theory of light. Particles, he argued in 1717, have sides and can thus exhibit properties that depend on the directions perpendicular to the direction of motion. It may be surprising that Huygens did not make use of the phenomenon of interference to support his wave theory; but for him waves were actually pulses instead of periodic waves with a certain wavelength. One should bear in mind that the word wave may have a very different conceptual meaning and convey different images at various times to different people. It took nearly a century before a new wave theory was formulated by the physicists Thomas Young of England and Augustin-Jean Fresnel of France. Based on his experiments on interference, Young realized for the first time that light is a transverse wave. Fresnel then succeeded in explaining all optical phenomena known at the beginning of the 19th century with a new wave theory. No proponents of the corpuscular light theory remained. Nonetheless, it is always satisfying when a competing theory is discarded on grounds that one of its principal predictions is contradicted by experiment. The corpuscular theory explained the refraction of light passing from a medium of given density to a denser one in terms of the attraction of light particles into the latter. This means the light velocity should be larger in the denser medium. Huygens' construction of wave fronts waving across the boundary between two optical media predicted the oppositethat is to say, a smaller light velocity in the denser medium. The measurement of the light velocity in air and water by Armand-Hippolyte-Louis Fizeau and independently by Jean-Bernard-Lon Foucault during the mid-19th century decided the case in favour of the wave theory (see Speed of light below). The transverse wave nature of light implied that the ether must be a solid elastic medium. The larger velocity of light suggested, moreover, a great elastic stiffness of this medium; yet, it was recognized that all celestial bodies move through the ether without encountering such difficulties as friction. These conceptual problems remained unsolved until the beginning of the 20th century.

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