Electromagnetic radiation is the transmission of energy in the form of waves having both an electric and a magnetic component. It is not possible for a wave with just one of these components to exist. The most familiar forms of electromagnetic radiation are radio waves and light waves. Less familiar forms are infrared radiation, ultraviolet light, X rays, and gamma rays, all of which constitute the electromagnetic spectrum. All of these forms are essentially the same physical phenomena, differing principally in the wavelength and frequency of the radiation. All electromagnetic waves propagate through empty space with the same velocity, c, equal to 299,792.4562 km/sec (186,282 mi/sec). For most calculations, the approximate value 300,000,000 m/sec is adequate. ELECTROMAGNETIC SPECTRUM Electromagnetic radiation is most simply characterized by its frequency or wavelength. When electromagnetic waves are ordered in accordance with their frequency or wavelength, this ordered array is called the electromagnetic spectrum. A source of radiation such as the Sun, a flame, an electrical discharge, or an incandescent solid never produces just one frequency of electromagnetic wave, but rather emits a mixture of waves of many different frequencies. These spectra may be resolved, or separated, by instruments such as prisms or grating spectrometers. In principle the electromagnetic spectrum extends from zero, the short wavelength limit of the gamma-ray end of the spectrum, to infinity, the long wavelength limit of the radio end of the spectrum. Visible light, the portion of the spectrum to which the eye is sensitive, occupies a narrow band extending from about 3.8/10,000,000 to 7.5/10,000,000 meters. The colored components of this segment of the spectrum may be seen when a narrow beam of sunlight is passed through a glass prism to form a band of colors that extends from red at the long wavelength end to violet at the short wavelength end. The radiation dispersed by the prism actually extends farther in both directions but light outside of this range cannot be detected by the human eye. SOURCES OF ELECTROMAGNETIC RADIATION The sources of electromagnetic radiation are accelerated electrical charges and oscillating currents. For example, the current that flows back and forth in the antenna of a radio transmitter radiates electromagnetic waves in the radio frequency part of the spectrum; these wavelengths lie in the range from a few centimeters to hundreds of meters. Similarly, the oscillatory motion of electrons in atoms and molecules radiates light waves with wavelengths that may be in the infrared region (1/1,000 to 1/1,000,000 meters), the visible region, the ultraviolet region (3/10,000,000 to 1/10,000,000,000/meters), or the X-ray region (1/100,000,000 to 1/100,000,000,000 meters). The wavelength of the emitted radiation depends on the energy of the oscillations that give rise to it. Visible and ultraviolet radiation arises when the outermost and least strongly bound electrons change their atomic energy levels. X rays are produced when atoms are bombarded by high-energy electrons that can strip a strongly bound electron from an orbit close to the nucleus. When an outer electron jumps into the vacancy, a high-energy \Tphoton\t (the particle equated with radiation) with a wavelength in the X-ray region of the spectrum is emitted. Gamma rays arise from transitions between energy levels of the nucleus. The nucleus is composed of protons and neutrons held together by nuclear forces that are much stronger than the electrical forces that bind the electrons in the atom. Because of the greater strength of the nuclear forces, the energy levels of the nucleus are widely separated, and the photons emitted as a consequence of nuclear transitions are more energetic than those emitted in atomic transitions. Gamma rays are also emitted in collisions between very high-energy charged particles such as cosmic rays colliding with nuclei of atoms of the atmosphere. The energy levels associated with the rotational and vibrational motion of molecules are closely spaced. Therefore, the photons emitted in transitions between these levels have small energies, and their wavelengths lie in the infrared part of the spectrum. Also, infrared radiation is emitted from solids, because the energy levels associated with lattice vibrations are so closely spaced as to virtually form a continuum. Radio waves are produced by coherent motion of electrons such as the oscillatory current that flows in the antenna of a radio transmitter. Since the early days of radio, the ingenuity of engineers has continually pushed the useful range of radio wavelengths down until today the lower limit lies in the millimeter range (microwaves), where it overlaps with the infrared spectrum. Radio waves are also produced by charged particles orbiting in magnetic fields. This is the source of much of the radio-frequency radiation observed by radio astronomers. Another important source of astronomical radio frequency radiation is a transition between two closely spaced energy levels in the hydrogen atom. In this transition the orientations of the spins of the electron and proton change from antiparallel to parallel with the emission of a photon of 21-cm wavelength. THE ELECTROMAGNETIC FIELD The theory of electromagnetic fields was developed by James Clerk \TMaxwell\t of Scotland and published in 1865. His work was the culmination of a long series of experiments and theoretical research performed by numerous illustrious scientists, including William \TGilbert\t, Benjamin \TFranklin\t, Charles Augustin de \Tcoulomb\t, Andre \TAmpere\t, Georg \Tohm\t, and Michael \TFaraday\t. (For a history of the development of electromagnetic theory, see \Telectricity\t.) Maxwell presented a set of equations that completely describe the electromagnetic field, how it is produced by charges and currents, and how it is propagated in space and time. The electromagnetic field is described by two quantities, the electric component E and the magnetic component B, both of which change in space and time. One of the solutions to Maxwell's equations is a plane wave traveling in the direction of the x-axis. If one examines a narrow region of space (fixed x) while the wave transverses it, the electric component oscillates in strength with the period one divided by the frequency. Examining the entire wave at any given instant (fixed t) reveals that the wave oscillates sinusoidally in space with the period l. Accompanying the electric component is a magnetic component. The amplitude of the oscillating magnetic component is equal to that of the electric component. B is perpendicular to both E and the direction of propagation. In addition, B and E are in phase; that is, they both are at maximum amplitude at the same time. It may be shown that electromagnetic waves transport energy as well as carry momentum. It may also be shown that any accelerated charge, not necessarily a sinusoidally oscillating one, loses energy in the form of electromagnetic waves. INTERACTION WITH MATTER Electromagnetic waves are modified as they pass through a material medium. The medium may be a solid, liquid, gas, or \Tplasma\t. (A plasma is an ionized gas, that is, a gas at a sufficiently high temperature so that the violent collisions of the atoms have dislodged one or more electrons from each atom.) As the wave propagates through the medium, each charged particle experiences a force that causes it to oscillate with the frequency of the wave. The oscillating charges modify the fields E and B. Consequently, the propagation characteristics of the wave are changed. One of these changes is simply the change in the velocity of propagation from the velocity in a vacuum c = 300,000,000 m/sec to the velocity v = c/n, where n is called the \Tindex of refraction\t and is characteristic of the medium. A consequence of this altered velocity is the phenomenon of \Trefraction\t. When a beam of light passes through a boundary separating two media of different indices of refraction (air and glass, for instance), the change in the velocity necessitates a change in the direction of propagation. The relation between these changes, known as Snell's law, (see \Trefraction\t) provides a means of experimentally measuring indices of refraction. It is the basis of much of the field of geometrical optics. Generally, the charged particles of a medium will respond differently to different frequencies, and as a consequence the index of refraction n will be a function of the frequency of the wave. If a beam of light that is a mixture of waves of different frequencies crosses a boundary between two media, the waves will be refracted through different angles. For example, if light from the Sun is reduced to a narrow beam and passed through a glass prism, the beam will be spread apart as it is resolved into its different spectral components. The visible light of longest wavelength, which the eye sees as red, is deviated the least, and the visible light of shortest wavelength, blue, is deviated the most. This phenomenon is known as \Tdispersion\t. In nonconducting gases, liquids, and solids the electrons are tightly bound in atoms and are only slightly displaced by the field of an electromagnetic wave. For such media the index of refraction is greater than, but close to, unity, the index of refraction of a vacuum. For example, for light in the visible part of the spectrum, n = 1.00029 for air, n = 1.333 for water, and n = 1.5 to 1.9 for glass. In good electrical conductors such as metals and plasmas, some electrons are free to move. Electromagnetic waves with frequencies less than a plasma's frequency, which depends upon its electron density, will not propagate in the medium. If such a wave is incident on the surface of such a medium, it will be reflected. This explains why most metals are good reflectors of visible light, although they may be transparent to higher frequency radiation such as X rays. One would expect that as the particles of a medium were set into motion in response to the field of a wave, they would experience frictional forces that would dissipate their energy and hence energy would be absorbed from the wave. This absorption does occur. The absorption coefficient (see \Tabsorption, light\t) depends on the frequency of the radiation. For some cases, such as waves propagating through plasmas, the viewpoint of frictional forces absorbing the energy is a fair approximation to reality. In these cases the friction is supplied by the collisions among the particles as they move in response to the waves. In other cases, such as light passing through gases and transparent solids, quantum theory (see below) must be invoked to explain the absorption process. In these cases absorption occurs when an electron can absorb the energy of a photon and jump from one energy state to a higher one. Then the absorption coefficient will be large at the value of the frequency for which the photon energy is equal to the energy difference between the states. QUANTUM ELECTRODYNAMICS The classical theory of electromagnetic radiation was the generally accepted theory at the beginning of the 20th century. The subsequent development of \Tquantum\t THEORY, however, has greatly modified the understanding of radiation. Many phenomena involve interference and diffraction that are most easily interpreted in terms of a wave theory of radiation. Others, such as the \Tphotoelectric effect\t, seem to require radiation to comprise corpuscles, which are now called \Lphoton\ls. The modern quantum theory of radiation, called \Tquantum electrodynamics\t, reconciles the wave and corpuscular pictures and explains both classes of phenomena. The energy E of a photon is equal to the product of Planck's constant h (a physical constant) and the frequency. The momentum p of a photon is equal to Planck's constant h divided by the wavelength. Thus Planck's constant connects the particle-like properties of energy and momentum and the wavelike properties of frequency and wavelength. According to the quantum theory of atomic structure, the energy of the atom cannot have an arbitrary value, as would be expected on the basis of classical theory, but rather is restricted to a set of discrete values, the energy states of the atom. Emission of radiation occurs when the atom makes a discontinuous transition from one of these energy states to a lower one. The energy is given to a photon whose frequency is determined by the conservation of energy principle. That is, the product of Planck's constant h and the frequency is equal to the difference in energies of the initial and of the final states of the atom. The atom can also absorb a photon and jump from a lower energy state to a higher one. Each atom has its own distinctive set of energy states, and consequently the presence of an atomic species can be recognized by examination of the emission or absorption spectrum. This has made \Tspectroscopy\t important for chemical analysis. Conversely, much has been learned about the structure of atoms by examining their spectra and from them deducing the set of energy levels that gave rise to them. More complex systems of charged particles such as molecules, solids, and liquids have their own peculiar systems of energy levels and consequently their own characteristic spectra. The study of the radiation emitted and absorbed by these systems has done much to further knowledge of their structures. Nuclei have their energy states just as atoms do, but because the nuclear forces that bind the protons and neutrons are so much stronger than the electrical forces that bind the electrons in atoms, nuclear energies are much greater. Therefore, the frequencies of the photons emitted by nuclei are much higher than those from atoms, and the wavelengths are very short, lying in the gamma-ray region below 1/10,000,000,000 meters. Much knowledge about the structure of nuclei has been obtained from the study of the gamma rays they emit. The importance that spectroscopy, the study of emitted and absorbed radiation, has played in modern physics cannot be overemphasized. The modern theory of electromagnetic radiation, quantum electrodynamics, can reasonably claim to be the most successful of all currently existing physical theories (with the possible exception of the theory of gravitation in Einstein's general theory of relativity). It explains with great accuracy the propagation of electromagnetic waves and their emission, absorption, and scattering by atoms, molecules, and solids. In addition it treats satisfactorily the creation and annihilation of pairs of electrons and positrons, their positively charged counterparts. The problems that remain in the theory concern the structure of the elementary particles themselves. Edward G. Harris Bibliography: Bekefi, George, Electromagnetic Vibrations, Waves, and Radiation (1978); Condon, E. V., and Shortley, G. H., The Theory of Atomic Spectra (1967); Jackson, John David, Classical Electrodynamics, 2d ed. (1975); Jordan, E. C., Electromagnetic Waves and Radiation Systems, 2d ed. (1968); Kraus, J. D., Electromagnetics, 3d ed. (1984); Lorrain, Paul, et al., Electromagnetic Fields and Waves, 4th ed. (1988).