From the History of Geometrical Optics

The History of Optics

The history of optics and optical devices begins in ancient Greece. The story of Archimedes, focusing the sun’s rays to win a battle for Syracuse in 213 BC is only a legend, reported centuries later. But in the Roman Empire, the philosopher, statesman and tragedian, Seneca noted the magnification of objects seen through water-filled transparent vessels, and his friend, the Emperor Nero, may have been the first to use a monocle, employing an emerald lens to view events in the Coliseum.

Spectacles, the first optical device, known also as eyeglasses, appeared first in Florence about 1280. The dispute exists over whether eyeglasses originated in the Far East or in the West: it appears that the eyeglasses used by the Chinese were for adornment or supposed magical powers and contained colored glass, not correcting lenses. And only in 1262 Roger Bacon, the medieval champion of experimental science, made the first recorded reference to the magnifying properties of lenses. In 1784 Benjamin Franklin invented bifocals. In his invention the two lens sections were hold by the frame. Johannes Kepler (1571 – 1630) was among the few to accept the Copernican heliocentric astronomy and he discovered the laws of planetary motion, which set the path for Newton’s theory of gravitation. In the course of his astronomical investigations he provided a correct explanation of vision and the functions of the pupil, cornea and retina and gave the first correct explanation of how eyeglasses work.

By 1610, Galileo Galileiannounced the telescopic observations of the moon and planets. One year earlier Galileo learned of the invention of the telescope by Hans Lippershey, who built a three-power instrument. His telescope wasa simple refractor, employing two lenses in a tube.Galileo quickly improved his telescope to eight, twenty and then thirty power. These were the most powerful instruments of his time.

But a man not only wanted to admire distant stars through telescopes, but to make closer some minor things. The invention of the compound (twin lens) microscope at the end of the sixteenth century or the beginning of the seventeenths has been ascribed to the Dutch spectacle maker, Hans Jansen. The first great improvement was due to Robert Hooke, who in 1665 replaced the eyepiece with the twin-lens telescope eyepiece designed by Christaan Huygens. Hooke’s three-lens microscope is the basis for modern instruments.

Sir Isaac Newton, a great scientist and thinker, who discovered some of the fundamental laws of mechanics, is known also by his invention of the reflecting telescope. Newton defended the idea of corpuscular nature of light, which implied that light consists of distinct particles with immutable properties.

The shift to the wave explanation of the nature of light began at the beginning of the 19th century. In 1801 Thomas Young discovered the interference of light from adjacent pinholes and established the wave theory of light. The polarization of light was discovered in1808 by Malus and the polarizing angle was discovered by Brewster in 1811. In 1842, an Austrian physicist Johann Christian Dopler published a paper "Concerning the Colored Light of Double Stars" which first described how the frequency of light and sound is changed by the relative velocity of the source and observer.

The union of electromagnetic theory with optics began when Maxwell found that his equations for the electromagnetic field (1873) described waves travelling at the velocity of light and with the demonstrations that electromagnetic waves were refracted and reflected like light waves. The final mathematical identification of optics with electromagnetism was achieved in 1944.

In the 20^th century revolutionary advances in optics began with the construction of the first laser in 1960 and have led to the rapid development of optical communication systems, imaging systems and holography, optical data storage and retrieval systems, and optical processing.

From the History of Geometrical Optics

An optical image may be regarded as the apparent reproduction of an object by a lens or mirror system, employing light as a carrier. An entire image is generally produced simultaneously, as by the lens in a camera, but images may also be generated sequentially by point-by-point scanning, as in a television system or in the radio transmission of pictures across long distances in space. Nevertheless, the final detector of all images is the human eye, and, whatever means is used to transmit and control the light, the final image must either be produced simultaneously or scanned so rapidly that the observer's vision will give him the mental impression of a complete image, covering a finite field of view. For this to be effective the image must be repeated (as in motion pictures) or scanned (as in television) at least 40 times a second to eliminate flicker or any appearance of intermittence.

To the ancients, the processes of image formation were full of mystery. Indeed, for a long time there was a great discussion as to whether, in vision, something moved from the object to the eye or whether something reached out from the eye to the object. By the beginning of the 17th century, however, it was known that rays of light travelled in straight lines, and in 1604 Johannes Kepler, a German astronomer, published a book on optics in which he postulated that an extended object could be regarded as a multitude of separate points, each point emitting rays of light in all directions. Some of these rays would enter a lens, by which they would be bent around and made to converge to a point, the "image" of the object point, whence the rays originated. The lens of the eye was not different from other lenses, and it formed an image of external objects on the retina, producing the sensation of vision.

There are two main types of image to be considered: real and virtual. A real image is formed outside the system, where the emerging rays actually cross; such an image can be caught on a screen or a piece of film and is the kind of image formed by a slide projector or in a camera. A virtual image, on the other hand, is formed inside an instrument at the point where diverging rays would cross if they were extended backward into the instrument. Such an image is formed in a microscope or a telescope and can be seen by looking into the eyepiece.

Kepler's concept of an image as being formed by the crossing of rays was limited in that it took no account of possible unsharpness caused by aberrations, diffraction, or even defocusing. In 1957 the Italian physicist Vasco Ronchi went the other way and defined an image as any recognizable nonuniformity in the light distribution over a surface such as a screen or film; the sharper the image, the greater the degree of nonuniformity. Today, the concept of an image often departs from Kepler's idea that an extended object can be regarded as innumerable separate points of light, and it is sometimes more convenient to regard an image as being composed of overlapping patterns of varying frequencies and contrasts; hence, the quality of a lens can be expressed by a graph connecting the spatial frequency of a parallel line object with the contrast in the image. This concept is investigated fully under optics and information theory.

Optics had progressed rapidly by the early years of the 19th century. Lenses of moderately good quality were being made for telescopes and microscopes, and in 1841 the great mathematician Carl Friedrich Gauss published his classical book on geometrical optics. In it he expounded the concept of the focal length and cardinal points of a lens system and developed formulae for calculating the position and size of the image formed by a lens of given focal length. Between 1852 and 1856 Gauss's theory was extended to the calculation of the five principal aberrations of a lens, thus laying the foundation for the formal procedures of lens design that were used for the next 100 years. Since about 1960, however, lens design has been almost entirely computerized, and the old methods of designing lenses by hand on a desk calculator are rapidly disappearing.

By the end of the 19th century numerous other workers had entered the field of geometrical optics, notably an English physicist, Lord Rayleigh (John William Strutt), and a German physicist, Ernst Karl Abbe. It is impossible to list all their accomplishments here. Since 1940 there has been a great resurgence in optics on the basis of information and communicationtheory.