Ibn al-Haytham

Lore & Background

Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham was born c. 965 in Basra, Iraq, then part of the Buyid emirate, to a family of Arab or Persian origin. His early influences were in religion and community service, but he later turned to mathematics and science. He held a position as vizier in Basra and became known for applied mathematics, including an attempt to regulate the flooding of the Nile. After moving to Cairo, he was given an administrative post but failed to fulfill it, incurring the anger of Caliph Al-Hakim. Legend says he feigned madness and was kept under house arrest until the caliph's death in 1021, during which time he wrote his influential Book of Optics. He continued living in Cairo near the University of al-Azhar, earning his living from writing and tutoring nobility, until his death c. 1040.

His most famous work, the seven-volume Kitab al-Manazir (Book of Optics), was written from 1011 to 1021. In it, he was the first to explain that vision occurs when light reflects from an object and then passes to the eyes, and to argue that vision occurs in the brain, pointing to observations that it is subjective and affected by personal experience. The work was translated into Latin by an unknown scholar at the end of the 12th or beginning of the 13th century, and later into Italian vernacular. It was printed by Friedrich Risner in 1572 under the title Opticae thesaurus. Risner also coined the Latin name variant 'Alhazen'; before him, Alhazen was known in the West as Alhacen.

Alhazen's theory of optics combined the mathematical ray arguments of Euclid, the medical tradition of Galen, and the intromission theories of Aristotle. He asserted that from each point of every colored body, light and color issue along every straight line. He argued that the eye only perceives perpendicular rays, using a physical analogy of a ball thrown directly at a board versus obliquely. He later asserted that other rays would be refracted and perceived as if perpendicular. His work directly or indirectly inspired much activity in optics between the 13th and 17th centuries, and Kepler's later theory of the retinal image built directly on Alhazen's conceptual framework.

Reader's Guide

Ibn al-Haytham's significance lies in his foundational contributions to optics and the scientific method. He was the first to correctly prove that vision is intromissive (light enters the eye from objects) rather than extramissive (the eye emits rays), and to argue that vision occurs in the brain, noting its subjectivity and dependence on personal experience. He stated a principle of least time for refraction, similar to Fermat's principle proposed in 1662. He made major contributions to catoptrics and dioptrics, studying reflection, refraction, and the nature of images. He was the first physicist to give a complete statement of the law of reflection, and the first to describe the horopter, spherical aberration, color constancy, and unconscious inference. He was an early proponent of the concept that a hypothesis must be supported by experiments based on confirmable procedures or mathematical reasoning, making him an early pioneer in the scientific method five centuries before Renaissance scientists; he is sometimes described as the world's 'first true scientist'. His Book of Optics was frequently cited during the Scientific Revolution by Galileo, Descartes, Kepler, and Huygens, and paved the way for the modern science of physical optics. He was also a polymath, writing on philosophy, theology, and medicine.

Did You Know?

• Ibn al-Haytham was the first to correctly prove that vision occurs when light reflects from an object and then passes to the eyes, and that vision occurs in the brain.
• He stated a principle of least time for refraction, similar to Fermat's principle proposed in 1662.
• He was the first physicist to give a complete statement of the law of reflection.
• He was an early pioneer in the scientific method, arguing that hypotheses must be supported by experiments based on confirmable procedures or mathematical reasoning.

Notable Quotes (Wikiquote)

"I constantly sought knowledge and truth, and it became my belief that for gaining access to the effulgence and closeness to God, there is no better way than that of searching for truth and knowledge." — Ibn al-Haytham "He moved to Egypt and supported himself by teaching and by copying Arabic translations of Greek mathematical classics such as Euclid’s Elements and Ptolemy’s Almagest." — Ibn al-Haytham