al-Hasan ibn al-Haytham

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al-Hasan ibn al-Haytham

The Arabian physicist, astronomer, and mathematician al-Hasan ibn al-Haytham (ca. 966-1039), or Alhazen, established the theory of vision that prevailed till the 17th century. He also defended a theory of the physical reality of Ptolemy's planetary models.

Al-Hasan was born at Basra in southern Iraq, where he must have received all his education. He gained sufficient fame for his knowledge of physics in his youth that he was called to Egypt by the Fatimid ruler al-Hakim to attempt to regulate the flow of the Nile. Failing in this effort, he was disgraced and established himself as a copyist of mathematical manuscripts; there still exists in Istanbul a manuscript of the Banu Musa's version of Apollonius's Conics copied by him in 1024. He continued to practice the scribal art in Cairo for the remainder of his life.

He did not cease to pursue his scientific studies, however, and published a large number of highly original works. He produced two catalogs of his own work, which are preserved by Ibn abi Usaybia. The first of these, compiled in 1027, comprises 25 books on mathematics and 44 on physics and metaphysics, including On the Structure of the World. The second, supplementary catalog was complied in 1028.

Work in Astronomy

The primary interest of al-Hasan was the explanation of phenomena by both mathematical and physical hypotheses. His interest in astronomy was motivated by the discrepancy between the Aristotelian physical and mechanistic model of the celestial spheres and the Ptolemaic mathematical model. On the Structure of the World, of which only the Latin translation has been published, describes the Aristotelian sublunar world of four elements and the Ptolemaic celestial spheres in all their complexity (his only change is to accept the theory that the solar apogee is fixed with respect to the fixed stars) as if they were material. He inserts a discussion of the perception of lunar and solar eclipses based on the assumption that the moon and sun are solid physical bodies.

This problem al-Hasan takes up again in On the Light of the Moon, in which he refutes the ancient theory that the moon reflects the sun's light like a mirror. Rather he believes that the moon is a self-illuminating body because each point on its surface broadcasts light rays in all directions, whereas each point on the surface of a mirror reflects a light ray from a single source (here the sun) in only one direction. However, he further believes that the eye receives two primary impressions in the act of vision: light and color. Therefore he concludes that only some physical effect of the sun's light rays on the moon renders the latter's color (and thereby its light) visible. This explanation opens the possibility of reconciling Aristotle and Ptolemy, for the element of which the heavenly bodies are constituted is now seen to be, though qualitatively unchangeable as Aristotle insisted, yet subject to some quantitative change which renders their light visible when they are struck by the sun's light.

Work in Optics

Al-Hasan's greatest scientific achievements were in the field of optics. In the discussion of the nature of vision at the beginning of Optics, he argues that light physically affects the eye, citing the pain experienced by looking directly at the Sun and the afterimage experienced by staring at fire and then looking into a weakly illuminated place. From this he argues that the assumption of emission of visual rays from the eye utilized by mathematical opticians, though convenient for their geometric analysis, must be physically wrong. Light rays rather proceed from the visible object to the eye and are always accompanied by color.

These mixed rays of light and color issue in all directions from a visible object, whether it is self-illuminating or an illuminated nontransparent body. They are perceived when the object lies in the visual field of the eye, each point on the surface of the visible object emitting a ray perpendicular to the front surface of the "glacial humor" (or "crystalline lens"); nonperpendicular rays are not perceived by the eye. The eye physically receives only the rays of light and color, but the mind interprets the patterns produced on the glacial humor as certain forms at certain distances. This theory of vision, after al-Hasan's book was translated into Latin in the late 12th or early 13th century, became the basis of all discussions of optics in the West until the 17th century.

In the latter part of Optics and in several other works, al-Hasan investigates problems of reflections from various sorts of mirrors. His famous solution is of "al-Hasan's problem, " which he encountered in examining spherically concave mirrors: given two points A and B on the plane of a circle with center O and radius R, find the point M on the circumference of the circle where a ray of light emitted from A must be reflected in order that it pass through B. This leads to a biquadratic equation which al-Hasan solves geometrically by the intersection of an equilateral hyperbole with a circle.

His Influence

Al-Hasan, who has been called, with some exaggeration, the founder of modern physics, seems certainly to have been the greatest Moslem student of physical theory, with the possible exception of the less well-known Qutb al-Din al-Shirazi. It is unjustified to be too emphatic about his originality until more is known about his predecessors. His contributions to science were, however, uniformly of the highest order.