(965 - 1040 C.E.)

Dr. A. Zahoor

Al-Haitham, known in the West as Alhazen, is considered as the father of modern Optics. Abu Ali Hasan Ibn al-Haitham was one of the most eminent physicists, whose contributions to optics and the scientific methods are outstanding. Ibn al-Haitham was born in 965 C.E. in Basrah (present Iraq), and received his education in Basrah and Baghdad. He traveled to Egypt and Spain. He spent most of his life in Spain, where conducted research in optics, mathematics, physics, medicine and development of scientific methods.

Al-Haitham conducted experiments on the propagation of light and colors, optic illusions and reflections. He examined the refraction of light rays through transparent medium (air, water) and discovered the laws of refraction. He also carried out the first experiments on the dispersion of light into its constituent colors. In detailing his experiment with spherical segments (glass vessels filled with water), he came very close to discovering the theory of magnifying lenses which was developed in Italy three centuries later. It took another three centuries before the law of sines was proposed by Snell and Descartes.

His book Kitab-al-Manazir was translated into Latin in the Middle Ages, as also his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena such as the rainbow, shadows, eclipses, and speculated on the physical nature of light. Roger Bacon (thirteenth century), Pole Witelo (Vitellio) and all Medieval Western writers on Optics base their optical work primarily on Al-Haitham's 'Opticae Thesaurus.' His work also influenced Leonardo da Vinci and Johann Kepler. His approach to optics generated fresh ideas and resulted in great progress in experimental methods.

Al-Haitham was the first to describe accurately the various parts of the eye and gave a scientific explanation of the process of vision. He contradicted Ptolemy's and Euclid's theory of vision that the eye sends out visual rays to the object of the vision; according to him the rays originate in the object of vision and not in the eye. He also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon. He is known for the earliest use of the Camera obscura. Through these extensive researches on optics, he has been considered as the father of modern Optics.

In Al-Haitham's writings, one finds a clear explanation of the development of scientific methods as developed and applied by the Muslims, the systematic observation of physical phenomena and their relationship to a scientific theory. This was a major breakthrough in scientific methodology, as distinct from guess work, and placed scientific study on a sound foundation comprising systematic relationship between observation, hypothesis and verification.

His research in catoptrics focused on spherical and parabolic mirrors and spherical aberration. He made the important observation that the ratio between the angle of incidence and refraction does not remain constant and investigated the magnifying power of a lens. His catoptrics contains the important problem known as Alhazen's problem. It comprises drawing lines from two points in the plane of a circle meeting at a point on the circumference and making equal angles with the normal at that point. This leads to an equation of the fourth degree. He also solved the shape of an aplantic surface for reflection.

In his book Mizan al-Hikmah, Al-Haitham has discussed the density of the atmosphere and developed a relation between it and the height. He also studied atmospheric refraction. He discovered that the twilight only ceases or begins when the sun is 19o below the horizon and attempted to measure the height of the atmosphere on that basis. He deduced the height of homogeneous atmosphere to be fifty-five miles.

Al-Haitham's contribution to mathematics and physics is extensive. In mathematics, he developed analytical geometry by establishing linkage between algebra and geometry. In Physics, he studied the mechanics of motion of a body and was the first to propose that a body moves perpetually unless an external force stops it or changes its direction of motion. This is strikingly similar to the first law of motion. He has also discussed the theories of attraction between masses, and it appears that he was aware of the magnitude of acceleration due to gravity.

Al-Haitham wrote more than two hundred books, very few of which have survived. His monumental treatise on optics has survived through its Latin translation. During the Middle Ages his books on cosmology were translated into Latin, Hebrew and other European languages. Also, he wrote a book on the subject of evolution; the ideas contained in that book are worth reading and useful even today.

Al-Haitham's influence on physical sciences in general, and optics in particular, has been held in high esteem and his ideas heralded in a new era in both the theoretical and experimental optical research. He wrote commentaries on Aristotle, Galen, Euclid and Ptolemy. Beer and Medler in their famous work Der Mond (1837) mention one of the surface features of the moon after Alhazen. It is the name of a ring-shaped plain to the west of the hypothetical Mare Crisium in Section No. 12.

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