Optics is the science concerned with the genesis and propagation of light, the changes that it undergoes and produces, and other phenomena closely associated with it. There are two major branches of optics: physical and geometrical. Physical optics deals primarily with the nature and properties of light itself. Geometrical optics has to do with the principles that govern the image-forming properties of lenses, mirrors, and other devices that make use of light. It also includes optical data processing, which involves the manipulation of the information content of an image formed by coherent optical systems.

The ancient Greeks and Arabs had some knowledge of the nature and properties of light. The foundations of the science of optics, however, were not established until the 17th century. During the early 1600s Galileo Galilei constructed the first telescopes that could be used for astronomical observation. In the 1650s the French mathematician Pierre de Fermat succeeded in deriving the law of refraction from a principle attributed to the Greek geometer Hero of Alexandria (1st century AD), according to which reflected light traverses the shortest distance between two points compatible with meeting the reflecting surface. By century's end, the Dutch mathematician-physicist Christiaan Huygens provided a mechanical explanation of reflection and refraction in his Traiti de la lumihre (1690; Treatise on Light). In the same work, he formulated a theory on the nature of light in which he related light to wave motion. In 1704 Isaac Newton published his Opticks, which included a comprehensive study of refraction, dispersion, diffraction, and polarization and a theoretical description of the corpuscular nature of light (i.e., light as consisting of moving particles). Newton's views, especially his particle theory of light, came to dominate scientific thought for over a century, completely overshadowing Huygens' contributions.

During the early 1800s Thomas Young, an English physician and physicist, studied the phenomenon of interference and found that it could only be explained if light consisted of waves. Young's findings, which were corroborated by the mathematical analysis of A.J. Fresnel of France, resurrected the wave theory of light. This conception held sway among the next several generations of investigators, including the British physicist James Clerk Maxwell, whose electromagnetic theory of light (1864) is generally considered the foremost achievement of classical optics. According to Maxwell's theory, light and various other forms of radiant energy are propagated in the form of electromagnetic waves--i.e., disturbances generated by the oscillation or acceleration of an electric charge and characterized by the temporal and spatial relations associated with wave motion.

The groundwork for modern optics was laid by the introduction of quantum theory at the turn of the century. The theory, proposed in 1900 by Max Planck of Germany, explained that radiant energy is emitted in discrete units, or quanta. In 1905 Albert Einstein extended this idea of light and demonstrated that, in the photoelectric effect, light behaves as though all of its energy were concentrated in minute particles later called photons. Einstein's finding, coupled with the electromagnetic theory, led to the present-day view that light behaves like waves in certain situations and like particles in others. The subsequent development of quantum mechanics, largely from 1925 to 1935, yielded a systematic explanation of this fundamental wave-particle duality of light.

Advances in physical optics were paralleled by rapid progress in geometrical optics. Lenses of reasonably high quality had been produced for telescopes and microscopes since the late 1700s, and in 1841 the German mathematician Carl Friedrich Gauss published his influential treatise on geometrical optics. In it he detailed the concept of the focal length and cardinal points of a lens system and devised formulas for calculating the position and size of the image formed by a lens of a given focal length. A little over a decade later, Gauss's theoretical work was extended to the calculation of the five principal aberrations of a lens (spherical, coma, astigmatism, Petzval field curvature, and distortion), thus establishing the basis for the formal procedures of lens design that were used for the next 100 years.

Two major developments, the emergence of communication and information theory in the 1950s and the invention of the laser in the early 1960s, ushered in a new era in optics. The initial tie between optics and communications arose because of the many analogies that exist between the two areas and because of the similar mathematical methods used to formally describe the behaviour of electrical circuits and optical systems. A matter of much concern since the invention of the lens as an imaging device has always been the description of the optical system that forms the image; information about an object is relayed and presented as an image. In this sense, an optical system can be considered a communication channel and be analyzed as such.

The manipulation of the content of an image by means of optical systems using coherent light (i.e., light of a single frequency or colour in which all the components are in step with each other) became a subject for serious study in the 1950s. The laser provided an ideal tool for optical data processing and communication. It gave rise to significant advances in holography, a two-step coherent image-forming process in which an intermediate record is made of the complex optical field associated with an object. A notable application of holography in the area of optical information processing is binary data storage and retrieval.

The laser, moreover, has proved to be a very efficient means of transmitting audio and video information (e.g., telephone conversations and television programs). It is superior to ordinary electronic transmitters for several reasons. Because the frequency of laser light is appreciably higher than that of radio waves, for example, a laser beam can carry substantially more information. Furthermore, since a laser beam is highly directional, it is able to transmit information with very little interference and over long distances. In long-distance communication applications, lasers are frequently used in conjunction with optical fibres of glass or plastic that make it possible to transfer laser light from one relay station to the next with almost no loss of energy.

Excerpt from the Encyclopedia Britannica without permission.