Ibn al-Haytham (Alhazen)

Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham was born around 965 in Basra, then part of the Buyid emirate. Some authors regard his family as Arab, others as Persian. His birth year is approximate; almost everything known about his life comes from later biographies, which contradict one another on certain points.

According to an autobiographical text from 1027 and later biographers, he doubted the competing religious currents of his time as a young man and chose mathematics and natural science instead. He is said to have held an administrative post in Basra and to have been known as a scholar before he left for Egypt.

His best-known work is the seven-volume Kitāb al-Manāẓir (Book of Optics), written between about 1011 and 1021. It treats light, colour, reflection, refraction and the structure of the eye. He argues that vision occurs because light from objects reaches the eye, rather than because the eye itself emits rays.

He supported his argument with experiments using mirrors, lenses and a darkened room with a small aperture (the camera obscura); according to the historian of science A. I. Sabra, the Optics shows for the first time a distinct concept of experiment. In 'Doubts Concerning Ptolemy' (about 1025–1028) he pointed to contradictions in Ptolemy's works and sought to improve Ptolemy's system rather than replace it.

In the centuries before him, Greek texts by authors such as Euclid, Ptolemy and Galen had been translated into Arabic, and scholars in Iraq and Egypt built on them. His Optics was strongly influenced by Ptolemy's Optics, while his description of the eye goes back to Galen; earlier Arabic authors such as al-Kindi also played a part.

Politically his world was divided: Basra fell under the Buyids, who effectively controlled the Abbasid caliphate in Baghdad, while Cairo had been the capital of the Shi'i Fatimid caliphate since the 970s. Caliph al-Hakim (996–1021) supported astronomers, but sources also describe him as unpredictable and violent.

In the Islamic world the Optics became known mainly through the commentary of Kamāl al-Dīn al-Fārisī, around 1300. Ibn al-Haytham also wrote on mathematics, astronomy and natural philosophy; biographers attribute more than two hundred works to him, of which just over fifty survive, some only in fragments.

Around 1200 an unknown translator rendered the Optics into Latin (De aspectibus). Scholars such as Roger Bacon, Witelo and later Johannes Kepler built on the work. Friedrich Risner published it in 1572 as Opticae thesaurus; he also popularised the name 'Alhazen', where 'Alhacen' had been customary before.

Ibn al-Haytham died around 1040, probably in Cairo. According to later biographers he lived after 1021 from teaching and copying texts. Stories about his Nile project, feigned madness and house arrest are not independently confirmed and appear in the sources in differing forms.

In modern literature he is often called the 'father of optics' or a forerunner of the experimental method; the International Year of Light (2015) commemorated his work on optics. Such titles were conferred later. Historians also point out that he built on Ptolemy, Galen and al-Kindi, and that his research must be understood within the scientific views of his own time.