Ibn al-Haytham

Ibn al-Haytham (c. 965 – c. 1040), known in Latin as Alhazen, was a mathematician, astronomer, and physicist from Iraq during the Islamic Golden Age. Often called the "father of modern optics," his most important work is the seven-volume *Book of Optics* (written between 1011 and 1021), which later survived in Latin translation. His writings were frequently referenced during the Scientific Revolution by figures such as Galileo Galilei, René Descartes, Johannes Kepler, and Christiaan Huygens.

He was the first to correctly demonstrate that vision is intromissive—light reflects from objects into the eyes—rather than extramissive, and argued that sight actually occurs in the brain, noting that it is subjective and shaped by personal experience. He also articulated a principle of least time for refraction, anticipating results later formalized in Fermat's principle (1662). In catoptrics and dioptrics, he studied reflection, refraction, and the nature of images formed by light rays. He gave the first complete statement of the law of reflection, and was the first to describe the horopter, spherical aberration, color constancy, and unconscious inference. An early advocate of testing hypotheses through experiments based on confirmable procedures or mathematical reasoning, he is sometimes considered the world's "first true scientist"—centuries before Renaissance scientists. A polymath, he also wrote on philosophy, theology, and medicine.

Born in Basra (then part of the Buyid emirate), he spent most of his productive years in Cairo, the Fatimid capital, earning his living by writing treatises and tutoring nobles. He is sometimes called by the byname "al-Basri" after his birthplace, or "al-Misri" ("the Egyptian"). Al-Haytham was dubbed the "Second Ptolemy" by Abu'l-Hasan Bayhaqi and "The Physicist" by John Peckham. His work paved the way for the modern science of physical optics.

His early life involved religious study and community service, but he eventually moved away from religion and into mathematics and science. He served as a vizier in Basra and became known for applied mathematics, once attempting to regulate the Nile's flooding. After returning to Cairo, he took an administrative post but failed at the task, angering Caliph Al-Hakim. According to legend, he feigned madness and was kept under house arrest until the caliph's death in 1021, after which his confiscated property was returned. During this period, he wrote his *Book of Optics*. He lived out his life in Cairo near the University of al-Azhar, supported by his writings, until his death around 1040. (A copy of Apollonius' *Conics* in his own handwriting, dated 1024, survives in Aya Sofya.) His students included Sorkhab (Sohrab), a Persian from Semnan, and Abu al-Wafa Mubashir ibn Fatek, an Egyptian prince.

The *Book of Optics* was translated into Latin by an unknown scholar around the late 12th or early 13th century, and later into Italian vernacular in the late 14th century. It was printed by Friedrich Risner in 1572 as *Opticae thesaurus*, who also coined the Latin name "Alhazen" (previously he was known as "Alhacen"). Works by Alhazen on geometric subjects were discovered in Paris in 1834 by E. A. Sedillot. In total, A. Mark Smith has identified 18 full or near-complete manuscripts and five fragments preserved in 14 locations, including the Bodleian Library and the library of Bruges.

Two major ancient theories of vision existed: the emission theory (Euclid, Ptolemy) held that the eye emits rays, while the intromission theory (Aristotle) held that physical forms enter the eye. Earlier Islamic writers like al-Kindi had followed Euclidean, Galenist, or Aristotelian lines. The *Book of Optics* was most strongly influenced by Ptolemy's *Optics*, while its description of eye anatomy and physiology drew from Galen.