(965-1040 A.D.)

Abu Ali Hasan Ibn al-Haitham was one of the most eminentphysicists, whose contributions to optics and the scientific methodsare outstanding . Known in the West as Alhazen, Ibn al-Haithamwas born in 965 A.D. in Basrah, and was educated in Basrah andBaghdad. Thereafter, he went to Egypt, where he was asked to findways of controlling the flood of the Nile. Being unsuccessful in this,he feigned madness until the death of Caliph al-Hakim. He alsotravelled to Spain and, during this period, he had ample time for hisscientific pursuits, which included optics, mathematics, physics,medicine and development of scientific methods on each of whichhe has left several outstanding books.

He made a thorough examination of the passage of light through various media and discovered the laws of refraction. He alsocarried out the first experiments on the dispersion of light into its constituent colours. His book Kitab-al-Manadhir was translated intoLatin in the Middle Ages, as also his book dealing with the coloursof sunset. He dealt at length with the theory of various physicalphenomena like shadows, eclipses, the rainbow, and speculated onthe physical nature of light. He is the first to describe accuratelythe various parts of the eye and give a scientific explanation of theprocess of vision. He also attempted to explain binocular vision,and gave a correct explanation of the apparent increase in size of thesun and the moon when near the horizon. He is known for the earliest use of the camera obscura. He contradicted Ptolemy'sand Euclid's theory of vision that objects are seen by rays of lightemanating from the eyes; according to him the rays originate in theobject of vision and not in the eye. Through these extensiveresearches on optics, he has been considered as the father of modern Optics.

The Latin translation of his main work, Kitab-al-Manadhir, exerted a great influence upon Western science e.g. on the work of Roger Bacon and Kepler. It brought about a great progressin experimental methods. His research in catoptrics centred on spherical and parabolic mirrors and spherical aberration. He madethe important observation that the ratio between the angle ofincidence and refraction does not remain constant and investigatedthe magnifying power of a lens. His catoptrics contain the importantproblem known as Alhazen's problem. It comprises drawing linesfrom two points in the plane of a circle meeting at a point on thecircumference and making equal angles with the norrnal at that point.This leads to an equation of the fourth degree.

In his book Mizan al-Hikmah Ibn al-Haitham has discussed the density of the atmosphere and developed a relation between it and the height. He also studied atmospheric refraction. He discoveredthat the twilight only ceases or begins when the sun is 19° below the horizon and attempted to measure the height of the atmosphereon that basis. He has also discussed the theories of attractionbetween masses, and it seems that he was aware of the magnitudeof acceleration due to gravity.

His contribution to mathematics and physics was extensive. In mathematics, he developed analytical geometry by establishing linkage between algebra and geometry. He studied the mechanicsof motion of a body and was the first to maintain that a bodymoves perpetually unless an external force stops it or changes itsdirection of motion. This would seem equivalent to the first lawof motion.

The list of his books runs to 200 or so, very few of which have survived. Even his monumental treatise on optics survived through its Latin translation. During the Middle Ages his books on cosmology were translated into Latin, Hebrew and other languages. He has also written on the subject of evolution a book that deserves seriousattention even today.

In his writing, one can see a clear development of the scientificmethods as developed and applied by the Muslims and comprisingthe systematic observation of physical phenomena and their linkingtogether into a scientific theory. This was a major breakthrough inscientific methodology, as distinct from guess and gesture, and placed scientific pursuits on a sound foundation comprising systematic relationship between observation, hypothesis andverification.

Ibn al-Haitham's influence on physical sciences in general, and optics in particular, has been held in high esteem and, in fact, it ushered in a new era in optical research, both in theory andpractice.

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from: http://www.oman-net.com/msp/haitham.htm