Without question, the greatest name in physics during the Arab/Islamic Empire was Ibn al-Haytham, born in the city of Basra, Iraq, in 965 A.D. By the time he died in 1030, he had made major contributions to optics, astronomy and mathematics, some of which would not be improved upon for six centuries.
Ibn al-Haytham's main field of interest and the one to which he made his greatest contributions, was the branch of physics we call optics. Striking parallels exist between his work and that of the seventeenth century English physicist, Isaac Newton, one of the greatest scientists of all time.
One of Newton's major accomplishments was his famous Law of Universal Gravitation. The most significant aspect of this theory is that it considers gravitation to be universal; that is, the same laws apply in the heavens and throughout the universe as apply on earth. This contradicts the idea held from the time of Aristotle (384-322 B.C.) that there is a difference between the laws governing events on Earth and those pertaining to celestial bodies. Newton realized that the force that causes an apple to fall from a tree is the same force that holds the moon and all the planets in their orbits and, indeed, is the same as that which governs the motion of the stars themselves.
If this idea were considered new in the seventeenth century, it was certainly new in the eleventh. Yet some of Ibn al-Haytham's experiments showed that he, too, believed that extraterrestrial phenomena obeyed the same laws as do earthly ones.
Ibn al-Haytham evolved his theories of optics through the study of light rays, and his investigations revealed a number of important properties: that light travels in a straight line; that every point of a luminous object radiates light in every direction; and that light weakens as it travels from its source. He studied these characteristics of light from a variety of light sources, i.e., self-emitting (the moon and reflecting bodies on earth).
This seemingly trivial experiment is in fact an early example of what is known as the "scientific method." Ibn al-Haytham designed an experiment to test a hypothesis, namely, that light travels in a straight line. His experiment was arranged to avoid the possibility of the experimenter's bias affecting the conclusions. Today, it seems obvious that light travels in straight lines, yet there was a time when intelligent men thought it obvious that the sun travels arounthe earth. The most advanced and sophisticated theory in modern physics, the Theory of Relativity, is derived from a refutation of ideas that are based on our everyday experience. Performing experiments to test and verify theories is at the heart of all modern scientific methods.
Ibn al-Haytham's experiments have even greater significance. By using the sun, the moon, lamps, fires and a variety of other light sources in his experiments, he was saying that light is light, regardless of its source. In this sense, he anticipated the universal laws of seventeenth century scientists.
We have described only the simplest of Ibn al-Haytham's experiments on the properties of light rays, but there are many others that were considerably more sophisticated. Ibn al-Haytham foresaw the works of later scientists not only in his use of experimentation but in the use of instrumentation: devices to help make measurements, the key to all modern science. He designed and constructed a variety of instruments, pipes, sheets, cylinders, rulers and plane, concave and convex mirrors in order to conduct his tests.
In addition to his studies of reflection, he also studied refraction, a phenomenon in which light rays bend when traveling from one medium to another, such as from air to water. The effect causes an object to appear to be in a location other than where it actually is, making him the first scientist to test a property of refraction that seems so obvious today. He demonstrated that a ray of light arriving perpendicular to the air-water boundary was not bent at all and showed that this was true for light passing through not just two, but several media. Clear parallels exist between his work and that of Isaac Newton six centuries later: both men studied that effects of light passing through glass, and both realized that the accepted ideas of their day were wrong.
It is difficult to appreciate the degree of intellect required by both these men to overcome the ingrained prejudices of previous centuries. The greatest scientists of Newton's day could not accept his theory of colors, a theory that we in the twentieth century, with three hundred years of hindsight, regard as self-evident. Newton's seemingly simple idea was that the colors produced when sunlight passes through a prism are caused by the separation of the sunlight, which contains all colors, into its constituent parts by refraction. Ibn al-Haytham demonstrated that the prism made the colors visible by bending rays of different colors in varying amounts, thus producing the familiar spectrum.
Ibn al-Haytham's explanation of how a lens worked required a similar leap of intellect. He contended that magnification was due to the bending, or refraction, of light rays at the glass-to-air boundary and not, as was thought, to something in the glass. He correctly deduced that the curvature of the glass, or lens, produced the magnification; thus, the magnifying effect takes place at the surface of the lens rather than within it.
This distinction is, of course, critical to the design of lenses, and without the ability to design lenses, we would have no cameras, movies, television sets, satellites, eyeglasses, contact lenses, telescopes, or microscopes—life would be very different for the human race.
Although he did not build a telescope, it is known that Ibn al-Haytham did construct parabolic mirrors. incoming parallel rays of light, such as those from the stars, are focused at a point so that such mirrors can be used to obtain unblurred images of celestial bodies and remote objects on the earth. Today, these are used in the world's great telescopes.
Like Newton, Ibn al-Haytham was interested in vision. Three Greeks, Galen in particular, did pioneering work on the anatomy of the eye and its connections to the brain, but did not produce a satisfactory theory of vision. Hero and Ptolemy both believed that vision was produced by the emission of light from the eyes, but their theory did not provide a reasonable explanation of perspective, the effect whereby the apparent size of an object depends upon its distance from the observer. As we know today, and as Ibn al-Haytham understood in the eleventh century, vision results from light being reflected into the eye from the object observed, an idea that explains perspective. He correctly regarded the eye as an intercepting screen, comparable to those we use today to show movies or slides. When his revolutionary ideas on perspective passed into Europe during the Renaissance, they influenced the development not only of science but also of art. The use of improved knowledge of perspective to give a feeling of depth and movement became strikingly visible in the works of Italy's new school of painters, the Perspectivi, around 1500.
Furthermore, Ibn al-Haytham appreciated that an explanation of vision must take into account not only such physical factors as light, screens, lenses and so on, but also anatomical and psychological factors, and he realized that the eye must function in a manner consistent with the laws of optics.
Ibn al-Haytham proved that the perception of an image occurs not in the eyes but in the brain and that the location of an image is largely determined by psychological factors. Like Newton, Ibn al-Haytham considered the problem of why a visual image produced within the brain is perceived as if it were located at some distance from the viewer, is the actual position of the object which produced it. Even today, most people do not find this surprising, although it is quite remarkable that images of the objects we see do not appear to be inside the head, where they actually exist, since they are simply electro-chemical versions of the scene inside the brain.
Ibn al-Haytham was aware of an even more subtle aspect of vision, namely, that when we see an object the brain automatically performs a memory retrieval procedure to see if it recognizes the object. The signals ultimately produced within the brain by light entering the eye cannot tell us that what we see is, for example, a loaf of bread. Almost instantly, the brain scans its memory and compares the new information it has received through the eyes with data it has stored over the years. Ibn al-Haytham called this function of the brain "the distinguishing faculty" and realized that it is intimately tied to the entire process of seeing.
That someone in the eleventh century realized that such complex questions existed is in itself noteworthy, but Ibn al-Haytham did not merely raise them, he attempted to provide answers. Explanations of these phenomena required him to construct a psychological theory of vision at a time when psychology was not recognized as a field of study. These ideas were quite different from the notions held by the Greeks and even by other contemporary Arab scientists.
The manner in which Ibn al-Haytham presented his theories in his Book of Optics is extremely interesting to the historian of science. He was both a mathematician and an experimenter, which allowed him to present his arguments with a power unmatched by previous scientists who rarely had experimental evidence to back up their assertions. Here lies another parallel between Newton and Ibn al-Haytham: they were both mathematicians and experimenters who made significant contributions to optics and other physical sciences by applying their knowledge of mathematics to the results of experiments. Ibn al-Haytham's descriptions of his experiments are replete with mathematical explanations in the form of geometric drawings, and he must have prepared engineering drawings or sketches to assist with the manufacture of his instrumentation.
About one-fourth of Ibn al-Haytham's more than 200 books and treatises survive; the best known of which is his Kitab al-Manazir, or Book of Optics (literally, Book of Perspectives). The breadth of the other subjects discussed in his book shows the wide range of his interests. They include optical illusions, the structure of the eye, binocular vision, perspective, atmospheric refraction, comets, mirages and the camera obscura. He is known to have studied physiology, anatomy and meteorology. Ibn al-Haytham also made notable contributions to astronomy. For example, he pointed out problems with the model of planetary orbits proposed by Ptolemy in the first century A.D., a model that was not superseded until the time of Copernicus in the sixteenth century.
It is not too much to claim that Ibn al-Haytham was not only the founder of the science of optics, but a pioneer in the modern scientific method and a man whose work stood unchallenged for six centuries before others were able to carry forward ideas that sprang from his fertile mind.
), by certain transformation on the field amplitudes, one can put the "classical" Hamiltonian in a form similar to that of infinite number of free oscillators. This makes life easy when we try to quantize the EM-field because, we know (from kindergarten QM) how to quantize oscillators.
