Alhazen, << ahl HA zehn >> (965-1040), also spelled Alhacen, an Arab scientist and mathematician, made influential contributions to the study of light and vision. His most important work, completed in the early 1000’s, was the massive Kitāb al-Manāzir (Book of Optics). In it, he claimed that vision results from rays of light passing from objects into the eye. At the time, most scholars accepted the view of the Greek thinkers Euclid and Ptolemy that the eye sends out visual rays to objects. But Alhazen said that a luminous or illuminated object gives off light rays from every point on its surface in all possible directions.
Alhazen thought that the front of the eye’s lens senses light. He claimed that it senses only those light rays that hit it at right (90°) angles and passes these rays into the eye. In this way, he thought, the eye forms images from the numerous rays striking it. Scholars accepted Alhazen’s theory, revised by later thinkers, from the late 1200’s to the early 1600’s. But in 1604, the German scientist Johannes Kepler showed that visual images form on a layer of tissue called the retina, rather than on the lens.
Alhazen was born in Basra, in what is now Iraq. He is known as Ibn al-Haytham in Arabic. The name Alhazen is a Latin form of al-Hasan, his first name in Arabic. Alhazen died in Cairo, Egypt.
See also Color (Early theories of color vision) .
