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Abu 'Ali al-Hasan ibn al-Hasan ibn al-Haytham

Abu 'Ali al-Hasan ibn al-Hasan ibn al-Haytham

965-c. 1040

Persian Astronomer, Mathematician, and Physicist

Al-Haytham was the greatest Arab scientist of the Middle Ages. His Kitab al-Manazir was the most important and influential work on optics between the time of Ptolemy (second century a.d.) and Johannes Kepler (1571-1630). Al-Haytham also made significant contributions to astronomy, mathematics, and medicine.

Al-Haytham, known to the Latin West as Al-hazen, was born in 965 in Basra (in modern Iraq). At an early age he became perplexed by the conflicting claims of competing religious sects. Frustrated by his failure to resolve these differences, he concluded that truth was only attainable through rational inquiry into empirical matters. However, he was unable to devote himself entirely to science. Hoping to attract the attention of the Fatimid caliph al-Hakam (996-1021) and to secure a more favorable position for himself, he claimed to have devised a means for regulating the flow of the Nile. Impressed, the dangerously unbalanced al-Hakim retained his services. However, it soon became clear that the project was hopeless. Al-Haytham admitted failure but, fearing for his life, feigned madness. Confined to his house, he maintained the ruse until the death of al-Hakim. In later years, he earned a living copying scientific and mathematical manuscripts.

Al-Haytham's theoretical and experimental investigations in optics surpassed all previous research in the field. In working out his radically new theories of light and vision, he articulated a comprehensive scientific methodology of the logical connections between observations, hypotheses, and verification. A distinctive feature of this methodology is its endorsement of experimentation through the manipulation of artificially created devices.

Kitab al-Manazir (Optica thesaurus) is al-Haytham's greatest work. At the time of its writing, theories of light were intimately connected with theories of vision based on the notion that vision required direct contact between the visual organ and objects of vision. Different accounts of how this contact occurred were promulgated and developed into opposing schools of thought in ancient Greece. Adherents of the intromission theory of vision believed objects emitted thin films or images of themselves through the intervening space to the eye. Exponents of the extromission theory believed the eye emitted an invisible fire that "touched" objects of vision to reveal their colors and shape.

In the Kitab, al-Haytham argued against the extromission view on the grounds that a material effluence flowing from the eye could not possibly fill the heavens fast enough to make vision possible. An insightful objection to traditional intromission theories had previously been provided by al-Kindi (801?-866?). Al-Kindi showed that if each point on a surface of an object radiates light in all directions, then each point on the eye will be stimulated by more than one point in the visual field, resulting in total confusion.

Al-Haytham's solution was a modified intromission theory. He formulated a comprehensive ray theory of light that allowed him to establish a one-to-one correspondence between each point on the eye's surface and points in the visual field. Specifically, he showed that only one ray was perpendicular to any given point on the eye's surface. All other rays would be refracted and thus weakened to point where they failed to stimulate the eye's visual power.

Al-Haytham also worked out the geometry of image formation in spherical and parabolic mirrors, observed that the angles of incidence and refraction do not remain constant, and conducted the first experiments on dispersion of light into its constituent colors. His reputation in mathematics derives from his sophisticated geometric analysis of mirrors, especially the "Alhazen problem," which involves finding the point of reflection for any two points opposite a mirror's surface.

In astronomy, al-Haytham noted that the Ptolemaic system failed to preserve uniform circular motion about Earth's center. Furthermore, he rejected the use of mathematical models whose motions could not be realized by physical bodies.

Al-Haytham correctly explained the increase in the apparent size of the Sun and Moon near the horizon in terms of atmospheric refraction. He also considered the atmosphere's density, developing a relationship between it and height. This allowed him to make an estimate of the atmosphere's height.

STEPHEN D. NORTON

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