Abu 'Ali al-Hasan ibn al-Haytham (Latin: Alhazen)

Mathematician and physicist, b. c. 965 (Basra, Iraq), d. 1039 (Cairo).

Al-Haytham, who is known to European science as Alhazen, was born and worked in Basra. His first training was for employment as a civil servant. He was appointed as a minister for Basra and the surrounding region.

In his autobiography al-Haytham recalls that as a young man he pondered the various religious movements and was unhappy with the uncertainty of truth in all religions. He decided to turn to science, which appeared to him more promising in that respect, and began by studying Aristotle. He quit his government position, and his reputation as an excellent scientist soon spread well beyond Basra.

When al-Haytham claimed publicly that he could construct a machine to regulate the annual floods of the Nile, he was ordered to the Fatimid court in Cairo. The caliph Al-Hakim was a great sponsor of the sciences and interested in science himself. But he was also a cruel and unpredictable ruler; he had, for example, ordered the killing of all dogs because their barking annoyed him. Looking at the Nile Al-Haytham soon realized that he had overestimated his capabilities. According to one report he managed to avoid execution by pretending to be mad and was kept under house arrest until al-Hakim's death in 1021. Other reports say that he fled to Syria or Baghdad. All agree that he worked in Cairo after 1021.

Throughout this period and until his death 18 years later al-Haytham worked on problems of physics, particularly optics, where he became the leading authority for centuries. His theory of viewing reversed the direction of the light ray taught be the Greek, who believed that the eye is an emitter of sensory matter.

Al-Haytham based his studies on the premise of the Indian scholar Varahamihira that the eye is a receptor of light rather than an emitter. This allowed him to develop a theory of reflection and refraction. In the introduction to his monograph Kitab al-Manazir he says that he will proceed by "criticizing premises and exercising caution in drawing conclusions" and "employ justice, not follow prejudice, and to take care in all that we judge and criticise that we seek the truth and not be swayed by opinions."

Following a discussion with the astronomer Abu al-Qasim ibn Madan about the apparent increase of the Sun's size at dawn and sunset, Al-Haytham turned his attention to problems of the atmosphere. His treatise Mizan al-Hikmah he discussed atmospheric refraction and discovered that twilight begins when the sun is 19 degrees below the horizon and attempted to measure the height of the atmosphere on that basis. He discussed the density of the atmosphere and developed a relation between it and the height. The work also contains comments on the attraction between masses which suggest that al-Haytham could be the first to have a had a concept of gravity.

Of al-Haytham's approximately 92 works on optics, astronomy, and mathematics over 55 have survived. The Polish physicist Witelo translated his Kitab al-Manazir into Latin in 1270 as Opticae thesaurus Alhazeni.

Al-Haytham's name lives on in modern physics in "Alhazen's Problem":

"Given a light source and a spherical mirror, find the point on the mirror were the light will be reflected to the eye of an observer."

The European scientist Huygens reformulated the problem again as:

"To find the point of reflection on the surface of a spherical mirror, convex or concave, given the two points related to one another as eye and visible object."