Ibn al-Haytham was the first scientist to maintain that vision occurs in the brain, not the eyes. By doing so, he pioneered what has become known as the psychology of visual perception. He argued that personal experience affects how and what people see. For example, a small child with little experience may have a hard time interpreting things he or she sees. At the same time, an adult can make mistakes in vision because experience suggests that he or she is seeing one thing, when really he or she is seeing another. Ibn al-Haytham was so fascinated by errors of vision that he devoted all of Book III to the topic. Perhaps it was this awareness—that vision and perception are more subjective than most people allow—that confirmed Ibn al-Haytham’s faith in a rigorous, skeptical approach to scientific inquiry.

Like other scholars such as Archimedes and Ptolemy, Ibn al-Haytham was fascinated by the effects that flat and curved mirrors have on light. He devoted Book IV to “reflection from smooth bodies,” Book V to “the forms seen inside smooth bodies,” Book VI to “errors in sight in what it perceives by reflection.” He also was interested in the way that transparent objects, such as water and glass, refract light. He devoted Book VII to “the manner of visual perception by refraction through transparent bodies.” In all four of these books, Ibn al-Haytham uses high-level geometry and mathematics to explain the behavior of light.

Book V contains one of the most enduring problems posed by ancient mathematics. Ibn al-Haytham imagined a scenario involving an observer, a light source, and a spherical mirror, all three in fixed locations. The observer looks upon the spherical mirror, which reflects the light from the light source. Ibn al-Haytham tried to determine the point on the spherical mirror where the light is reflected to the eye of the observer. The question had originally been formulated by Ptolemy in 150 A.D., but because Ibn al-Haytham considered it extensively, it is known as “Alhazen’s Problem” in the West. Ibn al-Haytham solved the problem using a geometric proof, but an algebraic solution to the problem eluded mathematicians until the end of the twentieth century.

Despite its brilliance, The Book of Optics is not free from errors. Many of Ibn al-Haytham’s findings were later proven wrong. Perhaps his greatest error was his failure to understand that the eye works like a small camera obscura, with the pupil acting as an aperture that projects a small image—upside and backwards—onto the part of the eye known as the retina. This discovery would not be made for another six hundred years, when a German scientist named Johannes Kepler accurately described how an image forms within the eye. Kepler and other scientists who proved portions of The Book of Optics wrong all did so by using the scientific method.

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Copyright © 2008 by Bradley Steffens

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