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Narrowly, the science of light and vision; broadly, the study of the phenomena associated with the generation, transmission, and detection of electromagnetic radiation in the spectral range extending from the long-wave edge of the x-ray region to the short-wave edge of the radio region. This range, often called the optical region or the optical spectrum, extends in wavelength from about 1 nanometer to about 1 millimeter. See Geometrical optics, Physical optics
The discoveries of the experimentalists of the early seventeenth century formed the basis of the science of optics. The statement of the law of refraction, the development of the astronomical telescope, observations of diffraction, and the principles of the propagation of light all came in this relatively short period. The publication of Isaac Newton's Opticks in 1704, with its comprehensive and original studies of refraction, dispersion, interference, diffraction, and polarization, established the science.
In the early 19th century many productive investigators established the transverse-wave nature of light. The relationship between optical and magnetic phenomena led to the crowning achievement of classical optics—the electromagnetic theory of J. C. Maxwell. Maxwell's theory, which holds that light consists of electric and magnetic fields propagated together through space as transverse waves, provided a general basis for the treatment of optical phenomena. In particular, it served as the basis for understanding the interaction of light with matter and, hence, as the basis for treatment of the phenomena of physical optics. See Electromagnetic radiation, Light, Maxwell's equations
In the twentieth century optics has been in the forefront of the revolution in physical thinking caused by the theory of relativity and especially by the quantum theory.
The science of optics finds itself in a position that is satisfactory for practical purposes but less so from a theoretical standpoint. The theory of Maxwell is sufficiently valid for treating the interaction of high-intensity radiation with systems considerably larger than those of atomic dimensions. The modern quantum theory is adequate for an understanding of the spectra of atoms and molecules and for the interpretation of phenomena involving low-intensity radiation, provided one does not insist on a very detailed description of the process of emission or absorption of radiation. However, a general theory of relativistic quantum electrodynamics valid for all conditions and systems has not been worked out.
The development of the laser has been an outstanding event in the history of optics. The theory of electromagnetic radiation from its beginnings was able to comprehend and treat the properties of coherent radiation, but the controlled generation of coherent monochromatic radiation of high power was not achieved in the optical region until the work of C. H. Townes and A. L. Schawlow in 1958 pointed the way. Many achievements in optics, such as holography and interferometry over long paths, have resulted from the laser. See Holography, Interferometry, Laser
the branch of physics that studies the nature of optical radiation, or light, and investigates light propagation and the phenomena observed during the interaction of light with matter. Since optical radiation consists of electromagnetic waves, optics belongs to the general study of electromagnetic fields. The optical wavelength band encompasses about 20 octaves and is bounded by X rays on one side and by the microwave radio-frequency band on the other. This demarcation is arbitrary and is largely due to the common character shared by the means used to investigate phenomena in the optical band. These means typically involve the formation of optical images of objects, based on the wave properties of the radiation, by devices whose linear dimensions are much greater than the radiation wavelength λ and the use of optical detectors whose operation depends on the quantum properties of light.
Optics traditionally has been subdivided into geometrical, physical, and physiological optics. Geometrical optics is not concerned with the problem of the nature of light. It proceeds from empirical laws governing light propagation. It uses the conception of light rays that are refracted and reflected at the boundaries of media having different optical properties and that are straight lines in an optically homogeneous medium. The task of geometrical optics is to investigate mathematically the path of light rays in a medium where the dependence of the index of refraction n on the coordinates is known or, conversely, to find the optical properties and shape of the transparent and reflecting media for which the rays are transmitted along a given path.
The methods of geometrical optics permit us to learn the conditions governing the formation of an optical image of an object as the aggregate of the images of the individual points of the object. They yield an explanation for many phenomena associated with the transmission of optical radiation in different media. The curvature of light rays in the earth’s atmosphere owing to the inconstancy of the index of refraction is an example of such phenomena; another example is the formation of mirages and rainbows. Geometrical optics, with the partial involvement of wave optics (see below), is of the greatest importance for the design and construction of optical devices—from eyeglasses to complex objectives and huge astronomical instruments. Thanks to the growth and the use of computer mathematics these design methods have become highly refined and a separate field called computational optics has appeared.
Photometry, which is devoted primarily to the measurement of light quantities, is also essentially unconcerned with the physical nature of light. The methods of photometry underlie the research on radiation emission, propagation, and absorption that examines the effects of the radiation on radiation detectors. Many photometric problems are solved by taking into account the principles governing the human eye’s perception of light and of the individual color components of light. These principles are studied by physiological optics, which borders on biophysics and psychology. Physiological optics investigates the optic analyzer, which extends from the eye to the cerebral cortex, and the mechanisms of vision.
Physical optics examines the nature of light and investigates light phenomena. The assertion that light consists of transverse electromagnetic waves is based on a vast number of experimental investigations of the diffraction, interference, polarization, and propagation of light in anisotropic media. The phenomena in which the wave nature of light is manifested are studied by a large branch of physical optics called wave optics. The general equations of classical electrodynamics—Maxwell’s equations—are the mathematical foundation of wave optics. Here the properties of a medium are characterized by macroscopic material constants—the dielectric constant ∊ and the magnetic permeability μ, which enter into are Maxwell’s equations as coefficients. These constants uniquely determine the index of refraction of a medium:
Phenomenological wave optics does not deal with the relation of the quantities ∊ and μ, which usually are known from experiment, to the structure of matter. It permits us both to explain the empirical laws and to establish the limits of applicability of geometrical optics. In contrast to geometrical optics, wave optics lets us consider processes of light propagation not only where the dimensions of the systems that form or scatter light beams are ≫ λ (the wavelength of the light) but for any ratio between the dimensions and the wavelength. In many cases the solution of concrete problems by the methods of wave optics turns out to be extremely complicated. There thus has been developed quasi optics, which describes the processes of propagation, refraction, and reflection in terms of geometrical optics but does not disregard the wave nature of the radiation. Quasi optics is particularly concerned with the longest wavelength region of the optical-radiation spectrum and the adjacent submillimeter subregion of the radio-frequency band. The geometrical and wave approaches are formally combined in the geometrical theory of diffraction, which postulates the existence of various types of diffracted rays in addition to the incident, reflected, and refracted rays of geometrical optics.
The establishment of the connection between the quantities ∊ and μ and the molecular and crystalline structure of matter played an immense role in the development of wave optics. It was now possible to go beyond a phenomenological description of optical phenomena and to explain the processes accompanying light propagation in scattering media, in anisotropic media, and near the interfaces of media having different optical characteristics. It was also possible to explain the dependence on λ of the optical properties of media, or dispersion, and the effect of pressure, temperature, sound, and electric and magnetic fields on light phenomena in different media.
In classical wave optics the parameters of a medium are considered to be independent of light intensity; optical processes are consequently described by linear differential equations. In many cases, however, particularly for high light flux intensities, this assumption has been found to be not valid. Moreover, new phenomena and new laws have been discovered. In particular, the dependence of the refractive index on the field intensity of the light wave—that is, the nonlinear polarizability of a substance—has a number of consequences. A number of examples are a change in the angle of refraction of a light beam at the interface of two media when the beam’s intensity is varied; compression and expansion of light beams, or self-focusing and self-defocusing of light; a change in the spectral composition of light transmitted through such a nonlinear medium, or the generation of optical harmonics; and the interaction of light beams and the appearance in the radiation of combination frequencies with separate directions of preferred propagation of the light, or parametric phenomena. All these phenomena are studied in nonlinear optics, a field that has developed with the advent of lasers.
Although it provides a good description of light propagation in material media, wave optics has been unable to give a satisfactory explanation of the processes of light emission and absorption. The investigation of these processes (for example, the photoelectric effect, photochemical transformations of molecules, and the laws governing optical spectra) and general thermodynamic considerations of the interaction of an electromagnetic field with matter have led to the conclusion that an elementary system such as an atom or a molecule can impart energy to the electromagnetic field or obtain energy from the field only in discrete amounts (quanta) that are proportional to the radiation frequency v. Thé electromagnetic field of light must therefore be equivalent to a quantum flux of light—photons that are propagated in a vacuum with the speed of light c = 2.99 1010 cm/sec. Photons have energy hv. The absolute value of their momentum is hv/c. Their mass is hv/c2, and their rest mass is equal to zero. They have spin h/2π. The quantity h = 6.65 X 10-27 erg/sec is Planck’s constant. In the simplest case the energy lost or acquired by an isolated quantum system upon interaction with optical radiation is equal to the energy of a photon. In a more complex interaction the energy is equal to the sum or difference of the energies of several photons. Phenomena in which the quantum properties of elementary systems are significant during the interaction of light with matter are the subject of quantum optics, which uses methods developed in quantum mechanics and quantum electrodynamics. Optical phenomena not associated with a change in the eigenstates of quantum systems—such as light pressure and the Doppler effect—can be explained in terms of either classical wave concepts or photon concepts.
The dual nature of light, that is, the simultaneous existence of features characteristic of both waves and particles, is one manifestation of the wave-particle duality that is inherent, according to quantum theory, in all objects of the microworld, including electrons, protons, and atoms. The concept of wave-particle duality was originally formulated for optical radiation and was conclusively established after the detection of wave properties such as diffraction in material particles. Only somewhat later was it experimentally confirmed for the band of electromagnetic radiation next to the optical band—the radio-frequency region, which in this connection is studied by quantum electronics or, more narrowly, quantum radio physics. The discovery of quantum phenomena in the radio-frequency region made the boundary between radio physics and optics much less sharp. In connection with the generation of induced radiation and the development of quantum mechanical amplifiers and quantum generators (masers and lasers) a new line of investigation appeared first in radio physics and then in physical optics. In contrast to the disordered light field of conventional—thermal and luminescent—sources, laser radiation is characterized by coherence, or ordering, because the field is controlled by the emission events of the elementary systems composing the laser. The radiation is distinguished by high monochromaticity (Δν/ν ∼ 10–13) and an extremely low beam divergence—down to the level of diffraction divergence. When focused, laser radiation permits the obtaining of radiation densities that cannot be achieved with other sources (∼1018 W·cm–2·steradian–1). The laser was a stimulus for reconsideration and development of the traditional branches of physical optics and for the appearance of new branches. Investigations of radiation statistics, or statistical optics, began to play an important role. New nonlinear and nonstationary phenomena were discovered. Methods of generating and controlling narrow coherent light beams—the field of coherent optics—were developed. Research on phenomena connected with the action of light on matter acquired particular importance, whereas before the advent of lasers the effects of matter on light had drawn the greatest attention. The growth of laser technology has led to a new approach in the construction of optical elements and systems and, in particular, has necessitated the development of new optical materials that transmit intense light fluxes without themselves being damaged; it has given rise to the field of power optics.
All branches of optics historically have had numerous practical applications. The problems of efficient lighting for streets, buildings, factory work areas, shows, historical and architectural monuments, and so on are solved by illumination engineering on the basis of geometrical optics and photometry, which takes into account the laws of physiological optics. The achievements of physical optics have found practical application in, for example, the designing of luminescent light sources. The achievements of optical technology are made use of in the manufacture of such devices as mirrors, light filters, and screens. One of the most important traditional problems of optics is the obtaining of an image that corresponds to the original both in geometric form and in brightness distribution (iconics). This problem is dealt with primarily by geometrical optics, but physical optics is also drawn upon for such purposes as determining the resolving power of instrumentation systems and taking into account the dispersion of light—the dependence of the refractive index on λ. Geometrical optics explains how to construct an optical system wherein each point of an object is represented by an image point and the geometrical similarity of the image to the object is preserved. Geometrical optics also shows the source of image distortions and indicates the level of such distortions in real optical systems. The processes used to manufacture optical materials—such as glass, crystals, and optical ceramics—with specified properties and the techniques used to finish optical elements play an important role in the construction of optical systems. Because of such production considerations, lenses and mirrors with spherical surfaces are used most often, although aspherical optical elements are also used to simplify optical systems and to improve image quality in systems with high aperture ratio.
The spatial distribution of the amplitudes and phases of the light waves propagated from a body is uniquely dependent on the body’s shape. This fact underlies holography, which offers new possibilities for optical imaging without the employment of focusing systems. In holography an additional coherent field is superposed on the field being recorded in order to take into account the phase distribution of the waves; the interference pattern that arises is fixed on a photosensitive layer or in other ways. When the hologram thus obtained is viewed in coherent monochromatic light, a three-dimensional image of the object is obtained. The development of sources of intense coherent light fields—lasers—initiated extensive work on holography. Holography has many scientific and technical applications. It is used, for example, to obtain three-dimensional images of objects, to record rapid processes under pulsed illumination, and to investigate displacements and stresses in bodies.
Optical phenomena and the methods that have been developed in optics are used extensively for analysis and monitoring in quite diverse fields of science and technology. Spectral and luminescence analysis methods, which are based on the relation of the structure of atoms and molecules to the character of the emission and absorption spectra and Raman scattering spectra of the atoms and molecules, are of particularly great importance. The molecular and atomic composition, state of aggregation, and temperature of a substance can be determined from the nature of the spectra and from the variation in the spectra with time or with the action of external factors on the substance. The kinetics of the physical and chemical processes occurring in the substance can be investigated on the basis of the same variables. The use of lasers in spectroscopy has resulted in the rapid growth of the new field of laser spectroscopy. Spectral and luminescence analysis are used in various fields of physics, astrophysics, geophysics, marine physics, chemistry, biology, medicine, and engineering, and in a number of the humanities and social sciences, including art studies and criminalistics.
The extremely high precision of measuring methods based on light interference has given such methods great practical importance. Interferometers are widely used in measuring wavelengths, studying the structure of spectral lines, determining the indexes of refraction of transparent media, making absolute and relative measurements of length, and measuring the angular dimensions of stars and other celestial bodies. In industry interferometers are employed to monitor the quality and shape of surfaces, to record slight displacements, to detect variations in temperature, pressure, or composition of a substance through small changes in the refractive index, and for other purposes. Laser interferometers with unique characteristics have been developed; because of the high power and high monochromaticity of laser radiation such interferometers have greatly increased the potential of interference methods.
A number of methods of investigating the structure of matter are based on the polarization of light; various polarization devices are used. The change in the degree of polarization or depolarization of light during scattering and luminescence can be the foundation for making estimates of, for example, thermal and structural fluctuations in matter, fluctuations in the concentration of solutions, intramolecular and intermolecular energy transfer, and the structure and arrangement of radiating centers. The polarization method of investigating stresses within and on the surface of a solid body has found broad application. In this method the mechanical stresses are determined from the change in the polarization of light reflected from or transmitted through the body. Polarization methods are used in crystal optics to study the structure of crystals, in the chemical industry to monitor the production of optically active substances, in mineralogy and petrography to identify minerals, and in optical instrument-making to increase the accuracy of instrument readings, for example, photometer readings.
Highly sensitive spectral instruments that have a diffraction grating as a dispersing element and make use of the phenomenon of light diffraction have come to be widely used. Examples of such instruments are monochromators, spectrographs, and spectrophotometers. Diffraction from ultrasonic waves in transparent media permits scientists to determine the elastic constants of a substance and to devise acoustooptical light modulators.
Optical methods involving the analysis of light scattering, especially by turbid media, have great importance for theoretical and applied molecular physics. Thus, nephelometry yields data on intermolecular interaction in solutions and permits determination of the dimensions and molecular weight of the mac-romolecules of polymers and of particles in colloidal systems, suspensions, and aerosols—information that is extremely important for atmospheric optics and the optics of dyes and powders. Valuable data on the energy structure of molecules and the properties of solids are provided by the Raman effect, Brillouin scattering, and induced light scattering, which have been developed by using lasers. The practical applications of devices based on optical quantum phenomena—such as photocells, photomul-tipliers, image brightness intensifiers (image converters), and camera tubes—are very numerous. Photocells are used not only to record radiation but also to convert the radiant energy of the sun into electric energy for powering electrical, radio, and other equipment (solar batteries). Photochemical processes underlie photography and are the subject of the special field of photochemistry, which links chemistry and optics. In addition to investigating the processes of intramolecular and intermolecular energy transfer, photochemistry pays considerable attention to the conversion and storage of light energy, for example, solar energy, and to the change in the optical properties of substances under the action of light (photochromy). New data recording and storage systems designed to satisfy the needs of the computer industry are being developed on the basis of photochromic materials. Such materials are also used in protective light filters that show an automatic increase in light absorption when the light intensity increases. The achievement of powerful fluxes of monochromatic laser radiation of different wavelengths has made possible the development of optical methods for isotope separation and for accelerating the directed course of chemical reactions. The achievement of such radiation has also permitted optics to find new, nontraditional applications in medicine and biophysics—involving, for example, the effect of laser light fluxes on biological entities at the molecular level. In engineering, lasers are made use of in optical methods of processing materials. The possibility of concentrating, by means of lasers, high radiation power on areas with linear dimensions of the order of tens of microns has led to intensive development of an optical method of producing a high-temperature plasma for the purpose of achieving controlled thermonuclear fusion.
The advances of optics have stimulated the development of optical electronics. The term “optical electronics’originally referred to the replacement of electronic elements in computing and other devices by optical elements. In the late 1960’s and early 1970’s there began to appear fundamentally new approaches to the problems of computer technology and data processing, approaches based on the principles of holography; at the same time, new applications for microoptical devices (integrated optics) were suggested. The advent of lasers brought new development to optical range-finding, optical location, and optical communication, which make extensive use of elements of control of light rays by electrical signals. The operation of many of these elements is based on the change in the character of light polarization when light is transmitted through electrically or magnetically active media. Optical range finders are used in applied geodesy, on construction projects, as altimeters, and in other capacities. A precise determination of the distance to the moon has been made through optical location. Optical location methods are being used to track artificial earth satellites, to transmit telephone conversations over laser optical communication lines, and to transmit images. The invention of light guides with low attenuation has resulted in the development of cable systems for optical videocommunication.
Virtually every field of science and technology makes use of optical methods. In many fields optics plays a dominant role.
Historical survey. Optics is one of the oldest sciences and has been closely connected with practical needs in all stages of its development. The linearity of light propagation was known to the peoples of Mesopotamia around 5000 B.C. and was made use of in ancient Egyptian construction projects. In the sixth century B.C., Pythagoras advanced the hypothesis that bodies become visible because of particles emitted by them; this hypothesis is not far from the modern theory. In the fourth century B.C., Aristotle claimed that light is an excitation of the medium between the object and the eye. He studied atmospheric optics and attributed rainbows to the reflection of light by drops of water. In the same century the two most important laws of geometrical optics—the linearity of light rays and the equality of the angles of incidence and reflection of light—were formulated in Plato’s school. Euclid’s treatises on optics in the third century B.C. dealt with the formation of images upon reflection from mirrors. The Greeks’ chief contribution, which constituted the first step in the development of optics as a science, lay not in their hypotheses as to the nature of light but in their discovery of the laws of light’s linear propagation and reflection, or catoptrics, and in their ability to make use of the laws.
The second important step in the development of optics was an understanding of the laws of light refraction, or dioptrics, and this came about only many centuries later. Dioptric experiments were described by Euclid and also by Cleomedes in the first century A.D. Aristophanes in about 400 B.C. and Pliny the Elder in the first century A.D. mentioned the use of glass spheres as kindling lenses. Ptolemy in A.D. 130 offered extensive information on refraction. At the time dioptrics was important primarily for its direct relation to the accuracy of astronomical observations. The laws of refraction, however, were not established by Ptolemy, or by Ibn al-Haytham, who wrote a celebrated treatise on optics in the 11th century, or even by Galileo or J. Kepler. But the empirical rules governing image formation by lenses were already well known in the Middle Ages, and the art of lens-making began to be developed at this time. Eyeglasses appeared in the 13th century. There is evidence that about 1590 Z. Janssen of the Netherlands constructed the first two-lens microscope. The first observations with a telescope, which was invented by Galileo in 1609, yielded a number of remarkable astronomical discoveries. The precise laws governing light refraction, however, were not experimentally established until later when W. Snell discovered Snell’s law about 1620 and R. Descartes set forth the laws in his “La Dioptrique” in 1637. This achievement and the subsequent formulation of the Fermat principle completed the foundation for the setting up and practical use of geometrical optics.
The further development of optics was associated with the discovery of light diffraction and interference by F. Grimaldi, who published his findings in 1665, and of double refraction by the Danish scientist E. Bartholin in 1669. These phenomena could not be explained by geometrical optics. Other important figures were I. Newton, R. Hooke, and C. Huygens. Newton paid great attention to the periodicity of light phenomena. He allowed the possibility of a wave interpretation but gave preference to a corpuscular conception of light. Newton considered light to be a flux of particles that act on the ether, in which they induce oscillations. Descartes introduced the term “ether” to designate a medium endowed with mechanical properties that is the carrier of light. According to Newton, the motion of light particles through an ether of variable density (variable because of the oscillations) and the particles’ interaction with material bodies are responsible for light refraction and reflection, the color of thin films, light diffraction, and light dispersion, which also was first studied in detail by Newton. Newton did not believe that light could be regarded as vibrations of the ether itself since attempts made at the time to explain the linearity of light rays and the polarization of light in such a way were unsuccessful. Polarization was first recognized by Newton although it followed from Huygens’ classical experiments on double refraction. According to Newton, polarization is a “primordial” property of light and can be attributed to a certain orientation of light particles with respect to the ray the particles form.
Following the ideas of Leonardo da Vinci and extending the work of Grimaldi and Hooke, Huygens proceeded from the analogy between many acoustical and optical phenomena. He believed that light excitation is pulses of elastic vibrations of the ether that propagate with a great, but finite, speed. It should be noted that Kepler and Descartes considered the speed of light to be infinite, while Newton and Hooke considered it finite. An experimental determination of the speed of light was first made in 1676 by O. Roemer. The Huygens-Fresnel principle, according to which each point of the front of a wave excitation can be regarded as a source of secondary, spherical waves, was Huygens’ greatest contribution to optics, a contribution that has not lost its value to the present day. The envelope, or surface, of the secondary waves is the front of the real propagating wave in the following moments of time. Relying on this principle, Huygens gave a wave interpretation of the laws of reflection and refraction. The correct expression for the index of refraction—n21 = v1/ v2, where v1 and v2 are the speeds of light in the first and second media—followed from his theory, whereas Newton and Hooke obtained the inverse ratio V2/V1, which does not conform to reality. Huygens also explained double refraction. When he spoke of light waves, Huygens did not ascribe literal meaning to them, and he did not use the concept of wavelength. He ignored the phenomenon of diffraction because he believed that light propagates linearly even through an arbitrarily small aperture. Huygens did not subject the phenomenon of polarization to examination. Nor did he mention Newton’s rings, which had been described in 1675. Newton’s rings are an interference effect that provides direct evidence of the periodicity of light oscillations and not of the pulsed character that Huygens assumed. Thus, although he formulated the fundamental principle of wave optics, Huygens did not work out a consistent wave theory of light that could stand comparison with Newton’s views. For this reason, and also because of Newton’s immense scientific authority, Newton’s corpuscular “theory of outflow” retained its dominant position until the early 19th century; its adherents gave it a categorical nature alien to the statements of Newton himself. Some prominent scientists, however, such as L. Euler and M. V. Lomonosov, did express a preference for the wave conception of light.
The foundation for the triumph of wave optics was laid by the works of T. Young and A. Fresnel. In 1801, Young formulated the principle of interference, which enabled him to explain the colors of thin films and provided a basis for an understanding of all interference phenomena. Fresnel made use of this principle to interpret the Huygens principle in a new way. He not only gave a satisfactory wave explanation of the linearity of light propagation but also explained many diffraction phenomena. Experiments by Fresnel and D. Arago established that waves with mutually perpendicular polarizations do not interfere. This finding permitted Young and Fresnel to advance, independently, the very important concept of the transverse nature of light oscillations. Fresnel used this concept to construct a wave theory of crystal-optical phenomena. Thus, all optical phenomena known by that time were provided with a wave interpretation. But there were difficulties even in this “triumphal procession,” since the detailed elaboration of the concepts of light as transverse elastic vibrations of the ether necessitated artificial theoretical constructs. The ether, for example, had to be endowed with the properties of a solid in which bodies nonetheless could move freely. These difficulties were basically resolved only with the systematic development of J. C. Maxwell’s theory of the electromagnetic field. Proceeding from M. Faraday’s discoveries, Maxwell concluded that light consists of electromagnetic rather than elastic waves. Later, in the early 20th century, the ether was found not to be necessary for propagation of the waves.
Faraday’s discovery in 1846 of the rotation of the plane of polarization of light in a magnetic field—the Faraday effect—was the first indication of a direct connection between electro-magnetism and optics. W. Weber and F. Kohlrausch established in 1856 that the ratio of the electromagnetic and electrostatic units of current intensity coincides in absolute magnitude and dimensionality with the speed of light c. Maxwell showed theoretically—and G. Hertz experimentally confirmed in 1888 —that changes in an electromagnetic field are propagated in a vacuum at the speed c. In a transparent medium the speed of light is that is, it is determined by the dielectric constant and the magnetic permeability of the medium. The initial attempts to explain the relationships then known between the index of refraction η and the radiation wavelength λ in terms of electromagnetic theory by using experimentally obtained values of ∊ and μ were unsuccessful. Normal dispersion—the increase in n with decreasing λ—had been known since Newton’s time. It had been explained from the standpoint of the elastic wave theory of light by Fresnel and A. Cauchy. But in 1862 the French physicist F. Leroux discovered a segment of the dispersion curve for which η increased with increasing λ. A. Kundt subsequently showed that such anomalous dispersion is inherent in many substances and is connected with their absorption of light. W. Sellmeier put forth in 1872 a conception of matter as a set of elastic oscillators, or resonators, with which light interacts. Developing this idea and examining the effect that forced oscillations of oscillators under the action of light have on the rate of propagation, H. Helmholtz in 1874 provided a complete theory of dispersion within the framework of the “elastic solid” theory of light. In the 1890’s, P. Drude, Helmholtz, and especially H. Lorentz combined the idea of oscillators and the electromagnetic theory of light to construct an electron theory of matter. Electrons were seen as components of atoms and molecules, within which they were capable of undergoing oscillations. This fruitful concept of the electron made possible a description of many optical phenomena, including normal and anomalous dispersion, since in the electron theory the value of ∊ depends on the frequency or wavelength of the electromagnetic field. D. S. Rozhdestvenskii’s very precise experiments on anomalous dispersion in 1912 yielded results that were in good agreement with the predictions of the electron theory. The discovery of the Zeeman effect—the influence of a magnetic field on the emission and absorption frequencies of atoms—in 1896 by P. Zeeman and the explanation of the effect in 1897 by Lorentz were a brilliant confirmation of the idea that the emission and absorption of light are determined by the behavior of electrons in atoms. The magnitude of light pressure, a concept first advanced in 1619 by Kepler to explain the deflection of comet tails away from the sun, also proved to be in complete agreement with Maxwell’s theory. Under terrestrial conditions the magnitude of the pressure was first measured by P. N. Lebedev in 1899. The construction of the electromagnetic theory of light and the supplementation of the theory by the electron theory of the interaction of light with matter constituted the next important step in the development of optics after the triumph of wave theory in the early 19th century.
The electromagnetic theory of light was the starting point for the creation of the theory of relativity. The experimental grounds for the development of the theory were certain data from optical experiments on moving media and on the motion of an observer with respect to the radiation source—data that contradicted the theoretical conceptions of the time. In 1804, Young showed that wave theory required an immobile ether not carried along by the earth in order to explain the phenomenon of the aberration of light. On the other hand, Fresnel found in 1818 that for the refractive index of bodies to be independent of their motion, as observed by Arago in 1810, the bodies must partially drag the ether along. This conclusion was reinforced by Fizeau’s experiment. In 1896, Lorentz developed the electrodynamics of moving media within the framework of the electron theory; the theory also led to partial entrainment of the ether. However, the classic Michelson experiment, which was first performed in 1881 and has since been repeated with increasingly high precision, did not detect this entrainment, or “ether wind.” The Michelson experiment, as well as other experiments inconsistent with the concept of a medium as the carrier of electromagnetic oscillations, found an explanation in the special theory of relativity formulated by A. Einstein in 1905. His theory led to a fundamental reexamination of many premises of classical physics and, in particular, finally eliminated the need for the ether—the hypothetical medium that was said to transmit light.
The classical Maxwell-Lorentz electrodynamic theory of light has repeatedly demonstrated its fruitfulness. In this connection there can be mentioned the explanation given by I. E. Tamm and I. M. Frank in 1937 for Cherenkov-Vavilov radiation, which had been discovered in 1934, and D. Gabor’s proposal of the idea of holography with the recording of a wave field in one plane. Another example is the development of the original approach to three-dimensional holograms that was initiated by Iu. N. Den-isiuk in 1962.
Despite its’ successes, electrodynamic theory was found to be clearly inadequate to describe the processes of light absorption and emission. Its inadequacy was manifested with particular sharpness by paradoxical conclusions, inconsistent with the law of conservation of energy, that the theory drew from analysis of the wavelength distribution of thermal radiation (black-body radiation). When he studied this fundamental problem, M. Planck in 1900 reached the conclusion that an elementary oscillatory system—an atom or molecule—imparts energy to an electromagnetic field or takes energy from it not continuously but in amounts proportional to the frequency of the oscillations —quanta. Planck’s assertion was at variance with classical concepts and introduced the idea of discontinuity, or discreteness, to light emission and absorption processes. Not only did Planck’s idea lead to a satisfactory solution of the problem of thermal radiation, its elaboration laid the foundation for modern quantum physics. Einstein in 1905 ascribed, besides energy, momentum and mass to light quanta, or photons. Thus the work of Planck and Einstein returned many features of corpuscular concepts to optics. In quantum optics the intensity of the electromagnetic field determines the probability of detecting a photon, and the structure of the field reflects the quantum structure of the ensemble of elementary radiators—atoms or molecules—and the distribution in time of emission events. In this way the physical meaning of the field is preserved, but the photons arising in light emission events and existing only in motion at the speed of light acquire characteristics of material particles. When a photon is absorbed it ceases to exist, and the system that absorbs it gains its energy and momentum. But the photon may not be absorbed upon interacting with a particle, such as a free electron, or it may be reflected from a macroscopic body, such as a stationary or moving mirror. In these cases the photon changes its energy and momentum in accordance with the laws governing the collision of two material bodies while preserving its absolute velocity. Photon concepts permitted Einstein to explain the fundamental laws of the photoelectric effect, which had been first investigated by A. G. Stoletov between 1888 and 1890, and to give a lucid treatment of photochemical conversions. The concepts made possible a clear explanation of a tremendous number of phenomena involving the interaction of light with matter, both phenomena known by the time quantum theory was formulated and phenomena discovered in subsequent years. For example, the existence of the shortwave limit in the bremsstrahlung spectrum of electrons—the maximum energy of the photon is equal to the energy of the electron; the Compton effect, which was discovered in 1922; the Stokes shift of the radiating frequency of photoluminescence with respect to the frequency of the exciting light; and the Raman effect, which was discovered in 1928 by L. I. Mandel’shtam and G. S. Landsberg and independently by C. V. Raman. The transition to quantum concepts was thus the next important step in optics. In its subsequent development optics cannot be considered in isolation from quantum physics in general.
In modern optics quantum concepts are not contrasted with wave concepts, but are combined in quantum mechanics and quantum electrodynamics. Quantum mechanics is of exceptional importance for spectroscopy, which has permitted scientists to obtain extensive information on the structure of atoms, molecules, and condensed media and on the processes that occur in those entities. These accomplishments have been made possible through the development of quantum theory in the works of N. Bohr, M. Born, E. Schrödinger, W. Heisenberg, W. Pauli, P. Dirac, E. Fermi, L. D. Landau, V. A. Fok, and many other physicists. Quantum theory, for example, has permitted scientists to interpret the spectra of atoms, molecules, and ions, to explain the effect of electric, magnetic, and acoustic fields on spectra, and to establish the dependence of the character of a spectrum on the conditions of excitation. An example of the reciprocal influence of optics on the development of quantum theory is the discovery of the intrinsic angular momentum, or spin, and the associated intrinsic magnetic moment of the electron (by S. Goudsmit and G. Uhlenbeck in 1925) and of other particles and of atomic nuclei. This discovery resulted from the necessity of explaining certain spectral features and entailed the establishment of the Pauli principle in 1925 and, in turn, the explanation of the hyperfine structure of spectra by Pauli in 1928. Thus, the setting up of the two most fundamental theories of modern physics—quantum mechanics and the special theory of relativity—was fostered above all by problems that arose during the development of optics and was based on the observation and analysis of optical phenomena.
A. Kastler’s discovery in 1953 of the optical orientation—that is, the orientation of the magnetic moments—of atoms by photons that impart their spin to the atoms when absorbed is an example of the advances of modern optics. The most important achievement of modern optics has been the detection of induced radiation of atoms and molecules, which was predicted by Einstein in 1916, and the development of methods of generating such radiation. A photon whose emission has been induced duplicates the photon that has caused the transition. If the supply of excited systems exceeds the number of absorbing systems, that is, if there is an active medium with a population inversion of the energy states of the atoms or molecules, this process can be repeated over and over. In other words, intensification of the initial light flux, the optical signal, occurs. The addition of optical feedback—for example, through the returning of part of the radiation by means of a mirror system—to such a quantum mechanical amplifier turns the amplifier into an optical quantum generator, or laser. The first quantum generators were masers—for the centimeter band—and were invented by A. M. Pro-khorov, N. G. Basov, and C. Townes in 1954 and 1955. The first ruby laser was constructed in 1960. Soon afterward, in the same year, the first gas-discharge laser, using a helium-neon mixture, was developed. Semiconductor lasers followed in 1962. The importance of this ground-breaking work was immediately appreciated, and there ensued numerous investigations of the properties of induced radiation and of ways of generating such radiation. It was established that lasers using solid, liquid, gaseous, and plasma media could be built through the use of various methods of population inversion. The advent of lasers stimulated the development of such traditional fields of optics as spectroscopy, luminescence, and photochemistry. The laser gave rise to entirely new fields of science and technology—nonlinear and parametric optics, power optics, and the optical processing of materials—and brought change to such fields already under development as optical communication and optical location. Through the use of lasers the practical realization and broad application of such earlier ideas as holography became possible. Lasers permitted scientists to extend optical methods to the solution of problems not previously in the purview of optics, for example, the problem of controlled thermonuclear fusion. Thus the invention of the laser attests to the dynamism of optics—a dynamism characteristic of sciences at the frontier of knowledge.
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