Ibn Al-Haytham

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His modern depiction on the obverse of the Iraqi 10,000 dinars banknote issued in 2003
Ibn al-Haytham or known to Europeans as Alhazen (965 in Basra - 1040 in Cairo) was a Muslim scientific thinker and polymath who made significant contributions to optics, physics, astronomy, mathematics, and the study of perception. Spending most of his life in Cairo, Alhazen argued that certain of Ptolemy's (90 - 168) earth-centered (geocentric) ideas in astronomy violated ideas of uniform circular motion and physical reality.

Alhazen was most original in his theory of light and vision, where he proceeded by experiment and induction, and again voiced skepticism of past authorities such as Euclid (300 BC) in his Optics whose ideas supported the extramission theory of vision. Alhazen rejected the classical concept of the visual cone emanating from the eye, pointing out the error in the assumption that some form of radiance could flow from the eye the moment it was opened and illuminate the entire heavens, as though the eye was like a torch sending fire to the object and making it visible (not far from the modern concept of radar).

Contents

Biography

Alhazen earned his living by transcribing and selling all the geometric works by Greek mathematician Euclid. Incidentally he developed a technical mastery of straight lines, motion and other Euclidian topics. Later he found employment at the court of the powerful caliph Al-Hakim in Cairo, who encouraged the enterprise of learning and technique as he obsessively wished to control all elements in the world around him. Desiring to command the Nile river, the caliph commissioned Alhazen to stop its annual flooding. Alhazen, though assuming the project, quickly concluded the undertaking's doom to failure and feigned insanity in order to escape the caliph's wrath. Unfortunately the plan did not succeed and he was sentenced to ten years in prison.

Under police control in miserly aphotic conditions Alhazen began to meditate on what he could see, becoming obsessed with light and dark. He questioned the classical idea, the accepted dogma for a thousand years, that sight was due to light coming out of a human eye and touching an object to produce vision (extramission). He countered that extramission could not explain vision because a person would experience severe pain in the eye when looking at a very bright object like the sun, when such pain should not exist had the eye been the source, not the recipient, of light. Though extramission could be refuted, very little evidence existed supporting intromission as it was very difficult to argue that a coherent, miniature version of an entire object could be received by the human eye, the nature of the shortening in the eye, and location of the corresponding projection.

Work in optics

Alhazen surmized that if light was independent of the eye, some mechanism had to redirect light into the eye. He realized there was a clue in the function of mirrors, claiming that light rays must be brought back (or make the object send back) some kind of visual packet to the eye. Studying those patterns, Alhazen uncovered the symmetry that the angle of incident and reflected light is equal (the angle at which light hits an object is the same as its reflection), analogous to the way a ball struck at, and rebounded from, a wall.

He made a breakthrough in realizing that light reflects off of all objects not just mirrors and precisely developed via mathematics how light bounces off (reflection) and bends through (refraction) and object. He validated the intromission hypothesis of vision by explaining light from luminous bodies is received by all other objects, which also become luminous. Alhazen was the first to employ the concept of rays of light travelling in straight lines from every point on its source in all directions, a critical advance on the classical theory that objects emitted coherent, tiny copies of themselves. In his improved model of vision, the eye was the recipient of light from the perceived object, creating a 'form' representation that enters the pupils, which act as lenses that bend light, and proceed to the brain, where sense completes the process.

In the year 1021, twelve years into his prison sentence, the caliph died and Alhazen was set free and produced seven volumes covering light and vision, transforming its mathematical rules and enacting the modern study of optics. His texts remained fundamental for centuries from the thirteen to the seventeenth centuries in the east and west, where Latin editions of Alhazen were studied by Kepler, Fermat and Descartes. Alhazen's book, Kitab al-Manazir or De Aspectibus or Book of Optics assimilated the physical, mathematical, and physiological aspects of vision.
Diagram of the eyes and related nerves, MS Illustration from Kitab al-Manazir (Book of Optics) by Ibn al-Haytham (Alhazen), Istanbul, Eleventh Century.

Other pioneering work

Alhazen virtually studied every aspect of light and human sight. He examined how light was bent (refracted) by water, bounced off (reflected) flat, smooth surfaces, and nearly converged on a theory of the magnifying lens. He examined the rainbow and explained the psychology of perception concerning the apparent moon diameter as the body approaches the horizon (an optical illusion caused by the expected size in relation to familiar objects on the ground), and demonstrated how refraction by the atmosphere causes the sun to be still visible when it is actually below the horizon. In order to study a solar eclipse, Alhazen cut a small hole in a wall, allowing the somewhat clear image to be projected through it onto a flat surface (an early example of the camera obscura).

Weaknesses

Unfortunately, Alhazen considered colors distinct from white light; he did not provide a satisfactory justification of how light refracted in the eye, but not through its center, is perceived.

See also

Notes

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