Ibn al-Haytham

Ibn al-Haytham, Latinized as Alhazen, in full, Abū ʿAlī al-Ḥasan ibn al-Haytham, (born c. 965, Basra, Iraq—died c. 1040, Cairo, Egypt), mathematician and astronomer who made significant contributions to the principles of optics and the use of scientific experiments.

Ibn al-Haytham’s most important work is Kitāb al-manāẓir (“Optics”). Although it shows some influence from Ptolemy’s 2nd century AD Optics, it contains the correct model of vision: the passive reception by the eyes of light rays reflected from objects, not an active emanation of light rays from the eyes. It combines experiment with mathematical reasoning, even if it is generally used for validation rather than discovery. The work contains a complete formulation of the laws of reflection and a detailed investigation of refraction, including experiments involving angles of incidence and deviation. Refraction is correctly explained by light’s moving slower in denser mediums. The work also contains “Alhazen’s problem”—to determine the point of reflection from a plane or curved surface, given the centre of the eye and the observed point—which is stated and solved by means of conic sections. Other optical works include Ḍawʾ al-qamar (“On the Light of the Moon”), al-Hāla wa-qaws quzaḥ (“On the Halo and the Rainbow”), Ṣūrat al-kusūf (“On the Shape of the Eclipse”; which includes a discussion of the camera obscura), and al-Ḍawʾ (“A Discourse on Light”).

In his Ḥall shukūk fī Kitāb Uqlīdis (“Solution of the Difficulties of Euclid’s Elements”) Ibn al-Haytham investigated particular cases of Euclid’s theorems, offered alternative constructions, and replaced some indirect proofs with direct proofs. He made an extended study of parallel lines in Sharḥ muṣādarāt Kitāb Uqlīdis (“Commentary on the Premises of Euclid’s Elements”) and based his treatment of parallels on equidistant lines rather than Euclid’s definition of lines that never meet. His Maqāla fī tamām Kitāb al-Makhrūṭāt (“Completion of the Conics”) is an attempt to reconstruct the lost eighth book of Apollonius’s Conics (c. 200 BC). Among his other mathematical works are treatises on the area of crescent-shaped figures and on the volume of a paraboloid of revolution (formed by rotating a parabola about its axis).

Ibn al-Haytham’s most famous astronomical work is Hayʾat al-ʿālam (“On the Configuration of the World”), in which he presents a nontechnical description of how the abstract mathematical models of Ptolemy’s Almagest can be understood according to the natural philosophy of his time. While this early work implicitly accepts Ptolemy’s models, a later work, al-Shukūk ʿalā Baṭlamyūs (“Doubts about Ptolemy”), criticizes the Almagest, along with Ptolemy’s Planetary Hypotheses and Optics.