Ibn al-Haytham

Ibn al-Haytham, known in medieval Europe as Alhazen, was a mathematician, physicist and astronomer of the Islamic Golden Age. Born in Basra in Iraq and active for most of his career in Fatimid Cairo, he wrote the Book of Optics, which explained vision as light entering the eye and tested its claims by systematic experiment. Translated into Latin, it shaped the study of light in Europe for centuries, and he is often called the father of modern optics.

Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham was born in Basra around 965, when the city was ruled by the Buyid dynasty. In an autobiography he described how, as a young man, he found the competing religious schools of his day unconvincing and turned instead to science, which he found best set out in the works of Aristotle. He is said to have held an official post in Basra, perhaps with the title of vizier, before giving it up for study, and he became known as an expert in applied mathematics.

His reputation reached the Fatimid Caliphate, which had conquered Egypt in 969 and founded Cairo as its capital. According to the medieval biographer al-Qifti, the caliph al-Hakim heard of his proposal to regulate the flooding of the Nile and invited him to Egypt to lead the work. As he travelled up the river, Ibn al-Haytham realised that the plan could not succeed, and he returned to report his failure. Given an administrative post instead, he came to fear the unpredictable caliph and, the story goes, pretended to be mad, living confined to his house until al-Hakim's death in 1021. Other accounts differ in detail.

During these years he wrote his greatest work, the seven-volume Kitab al-Manazir, or Book of Optics, composed between about 1011 and 1021. Ancient thinkers had disagreed over whether sight came from rays sent out by the eye, as Euclid and Ptolemy held, or from forms entering it, as Aristotle taught. Ibn al-Haytham combined the geometry of rays, the medical study of the eye and the idea that light enters from outside into a single theory: light travels in straight lines from every point of a lit object, and vision happens when it reaches the eye, with the brain completing the perception.

He tested these ideas with experiments using lenses, mirrors and screens, varying conditions step by step. He studied reflection and refraction, built an apparatus to measure angles of refraction, and gave an early clear account of the camera obscura, using a small opening to watch the crescent-shaped image of the sun during a partial eclipse. He also explained colour constancy and argued that the Moon looks bigger near the horizon because of how the mind judges distance.

After al-Hakim's death Ibn al-Haytham lived near the al-Azhar mosque in Cairo, supporting himself by writing, teaching and copying texts. In his Doubts Concerning Ptolemy, written between about 1025 and 1028, he criticised contradictions in Ptolemy's astronomy and insisted that models of the heavens must describe physically possible arrangements. A later planetary model, written around 1038, kept the Earth at the centre but did without Ptolemy's equant. He also worked on geometry and number theory. Estimates of his output range from about 90 to more than 200 works, of which more than 50 survive at least in part. He died in Cairo around 1040.

The Book of Optics was translated into Latin in the late 12th or early 13th century and printed in 1572. It was read by Roger Bacon, Witelo and other medieval scholars, and later by Johannes Kepler, whose theory of the image on the retina built on its framework. His insistence that authorities be questioned and claims tested by experiment and mathematics has led some historians to see him as a pioneer of scientific method, though others caution against viewing him in isolation from Greek and earlier Islamic scholars.

Our 1000 map shows the two states in which he lived and worked: the Buyid lands around Basra and the Fatimid Caliphate in Egypt.