The Arabs who were pioneers in diverse branches of mediaeval sciences have made invaluable contribution to the field of optics. Of all the sciences that have received the imprint of Arab genius, optics stands on the top. The highest authority in optics during the mediaeval times whose brilliance outshone that of the Greek thinkers was Ibn Al-Haitham, better known as Alhazen in the West. “The glory of Muslim science,” writes Max Meyerhof, “is in the field of optics. Here the mathematical ability of Alhazen and Kamal-a-Din outshone that of Euclid and Ptolemy. Real and lasting advance stands to their credit in this department of science”.

Abu Ali Al Hasan ibn Al-Haitham was one of the most outstanding mathematicians, physiologists and opticians of Islam. He was born at Basrah in 965 A.C. and received his education in Basrah and Baghdad which were great centres of learning in those days. Later, he moved to Egypt and entered into the service of the celebrated Fatimid Caliph Al-Hakim (996-1020 A.C.) and was entrusted with the task of discovering the method of controlling the annual floods caused by the Nile. But, having been unsuccessful in this mission, he remained underground and also assumed madness till al-Hakim’s death. He also visited Spain. During this period he got ample time for his intellectual pursuits and composed a number of works on medicine, which was his original profession. He died in Cairo in 1039 A.C.

Alhazen, a versatile genius, made lasting contributions to optics, mathematics, physics, medicine and philosophy. The author of ‘Uyunul Ariba fi Tabaqaat il Atibba’, a well-known biographical work quoted the names of 200 scientific works written by Alhazen on diverse subjects, including optics, mathematics, medicine, physics and philosophy. Alhazen is best known to Europe through his optical works, which were translated into Latin.

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His main work on optics, which was lost in Arabic, still survives in Latin. In it, Alhazen has corrected the conception of Ptolemy and Euclid that the eye sends out visual rays to the object of vision. “He was the first to correct the Greek misconception as to the nature of the vision”, observes John Wil: am Draper in his History of the Intellectual Development of Europe, “showing hat the rays of light come from external objects to the eye, and do not issue forth from the eye, impinge on external things, as, up to his time, had been supp..sed. His explanation does not depend upon mere hypothesis or supposition, but is plainly based upon anatomical investigations as well as geometrical discussion”.

His works on optics influenced the greatest Western scientists and paved the way for later discoveries and developments in the field of optics. He has examined the refraction of light rays through transparent objects, including air and water. In experimenting with spherical segments (glass vessels filled with water) he has forestalled the theoretical discovery of magnifying lenses, which was made three centuries later in Italy. He also prepared the ground for Snell and Descartes to establish the law of sines, six centuries later.

Among other things he discussed was the “propagation of light and colors, optic illusions and reflection, with experiments for testing the angles of incidence and reflection”. His ‘Alhazen Problems’ in optics are still known throughout the West. In a spherical concave or convex, a cylindrical or conical mirror to find the point from which an object of given position will be reflected to an eye of given position. It leads to an equation of the fourth degree which Alhazen solved by the use of a hyperbola.

He made a number of monumental discoveries in the field of optics, including one which locates retina, as the seat of vision. This discovery falsified the earlier contention of the Greek scientists that eye sends out visual rays to the object of the vision.

According to him, the impression made by light on “retina is conveyed to the brain through optic nerves. He also explained how only one object is visible when seen from both the eyes because the visual images are formed on symmetrical portions of the two retinas. The optical illusions, according to him, are due to rays of light suffering from reflection or refraction. “He is perfectly aware”, writes a Western orientalist, “that the atmosphere decreases in density with the increase of height, and from that consideration he shows that a ray of light, entering it obliquely, follows a curvilinear path which is concave toward the earth, and that, since the mind refers the position of an object to the direction in which the ray of light from it enters the eye, the result must be an illusion as respects the starry bodies; they appear to us, to use the Arabic term, nearer to the zenith than they actually are, and not in their true place.” He has found out a wonderful optical illusion in the twilight when he sees stars. Sun and Moon before they have risen and after they have set. He has explained that greater the density of atmosphere, the greater the curvature of a ray of light. “To this refraction, he truly refers the shortening, in their vertical diameter, if the horizontal Sun and Moon; to its variations, he imputes the twinkling of the fixed stars”. He quite accurately determined the height of the atmosphere as nearly 58% miles. Describing his lasting contribution to optics, John William Draper states; “All this is very grand. Shall we compare it with the contemporary monk miracles or the monkish philosophy of Europe? It would make a profound impression if communicated for the first time to a scientific society in our own age”. Commentaries on Alhazen’s optics were written by oriental writers and his views were to some extent shared by Al-Biruni and Avicenna. His book on optics ‘Kitab Al Manazir’ was translated into Latin by F. Risner and published in Basle in 1572 A.C. His other work on twilight was translated into Latin and published in the same year. His optics has immensely influenced the Western scientists, including the celebrated Roger Bacon and Vitello.

Al Hazen has written several treatises on physical optics, including one on light. He thinks that light is a sort of fire which is reflected at the spheric limit of the atmosphere. In another treatise entitled, ‘On Twilight Phenomena’, he has calculated this atmosphere to be about 10 miles in height. Some of his treatises on physical optics deal with the halo, the rainbow, and with spherical and parabolic mirrors. He has fully explained the factors which cause a rainbow. He has also written treatises dealing with shadow and eclipses. All of these invaluable treatises are of a highly mathematical character. “Most of his works were products of the last ten years of his life”, writes Max Meyerhof, “as was his fundamental study on the burning glass, in which he created a dioptric far superior to that of the Greeks. The work exhibits a profound and accurate conception of the nature of focussing, magnifying, and inversion of the image, and of the formation of rings and colours by experiments.” The Muslim scientists were always particular in supporting their hypothesis with experiments. So was the case with Alhazen. During lunar eclipses, Alhazen observed the semi-lunar image of the Sun on a wall which was opposite to a small hole made in the window shutters. This was the first record of the camera obscura. Alhazen has also written commentaries on the optical works of Ptolemy and Euclid and on the physics and problemata of Aristotle.

His ‘Mizanul Hikma’ deals with the density of the atmosphere in which he has established a connection between the height of the atmosphere and its increasing density. He demonstrated that the weight of body increases in proportion to the increasing density of the atmosphere. He explains the force with which bodies will rise out of heavier media, in which these are immersed and also “discussed the submergence of floating bodies as ships upon the sea”.

He has discussed the problem regarding the Centre of Gravity and has successfully applied it to the investigation of balances and steelyards, explaining the relations between the centre of gravity and the centre of suspension. He recognised gravity as a force, a theory which was later on developed by Newton. He knows correctly the relation between the velocities, spaces and time of falling bodies, and has very distinct idea of capillary action. “The determination of the densities of bodies”, observes a Western writer, ‘as given by’ Alhazen, approach very closely to our own; “in the case of mercury they are more exact than some of those of the last century.”

His treatise on ‘Configuration of the Universe’ was translated into Latin. A Hebrew translation of it was made by the Jew Jacob bin Mahir, which was again translated into Latin by Abraham de Balmes in the latter half of the 15th century. This book was again translated in Spanish by Abraham de Toledo in the latter half of the 13th-century A.C.

The optical works of Alhazen had a deep influence on the mediaeval scientists both in the East and the West. His writings on the subject paved the way for later researches on optics in the West. His works on optics and treatises on physics were translated into several European languages. A book on the balance of Wisdom said to be written by Alhazen, translated by M. Khanikoff, the Russian Consul General at Tabriz, contained some useful mechanical theories and conceptions which have been propounded with exceptional clarity. About his views on human evolution, John William Draper observes: “Though more than seven centuries part him from our times, the physiologists of this age may accept him as their compeer, since he received and defended the doctrine now forcing its way, of the progressive development of animal forms.” He upheld the affirmation of those who said that man, in his progress, passes through a definite succession of states; not, however, “that he was all once a bull, and was then changed to an ass, and afterwards into a horse, and after that into an ape, and finally became a man.” “This,” he says, “is only a misrepresentation by people of what is really meant.” His progressive ideas on the evolution of man led to the development of this theory by modern scientists, including Darwin.

The abbreviated translations of a large number of Alhazen’s treatises have been published in German, by E. Weidemann, together with critical notes. The celebrated American historian George Sarton has given a large list of his works in his Introduction to the Study of Science! According to Phillip K. Hitti, “Roger Bacon, Leonardo da Vinci and John Kepler, show traces of his influence. His ‘Kitab Al Manazir’ (The Book on Optics) influenced all the mediaeval writers on the subject, who borrowed their ideas from him.” “Roger Bacon” (13th century), admits Max Meyerhof, “and all mediaeval writers on optics-notably the Pol Witelo or Vitello, base their optical works largely on Alhazen’s Optical Thesaurus. His work also influenced Leonardo da Vinci and John Kepler. The latter modestly entitled his fundamental work on dioptrics Ad Vitellionem Paralipomena (Frankfurt 1604).

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