Ibn al-Haytham may have written his Book of Optics while feigning madness
Legend says that after failing a task for Cairo's volatile caliph al-Hakim, Ibn al-Haytham pretended to be mad and lived under house arrest. In those years he produced the seven-volume Book of Optics, which proved that we see because light enters the eye. Galileo, Kepler, Descartes and Huygens all later cited him.
Known in Latin as Alhazen, he was born in Basra around 965 to a family of Arab or Persian origin. Disillusioned by the clashing religious views of his time, he turned to mathematics and science. He held the title of vizier in Basra and became known for applied mathematics, including an attempt to regulate the Nile's floods. In Cairo he failed at an administrative post, angered al-Hakim, and is said to have hidden until the caliph died in 1021, when his confiscated property was returned. He spent the rest of his life near al-Azhar, living off his writing until about 1040.
Antiquity had two rival theories of sight: Euclid and Ptolemy held that the eye emits rays, while Aristotle's followers held that forms enter the eye. Ibn al-Haytham, writing between 1011 and 1021, was the first to show correctly that vision happens when light reflects off objects into the eye, and he argued that seeing takes place in the brain, since perception is subjective and shaped by experience. He demonstrated by experiment that light travels in straight lines, gave the first complete statement of the law of reflection, and proposed a least-time principle for refraction resembling the one Fermat stated in 1662.
His work on binocular vision anticipated later discoveries. Ian P. Howard argued in 1996 that he described what became Hering's law and the vertical horopter centuries before Europeans; others, such as Raynaud, add nuance, noting he lacked any understanding of the retina's role. He also described spherical aberration and colour constancy.
An unknown scholar translated the Book of Optics into Latin around 1200, and Friedrich Risner printed it in 1572, coining the name Alhazen. Medieval writers called him the Second Ptolemy and simply The Physicist, and a copy of Apollonius's Conics in his own hand survives in Istanbul. Today he is often called the father of modern optics.
Source: Ibn al-Haytham
