Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham (c. 965–1040) is known in Latin as Alhazen. Born in Basra, he spent the productive part of his adulthood in the Cairo of the Fatimid caliph al-Hakim. Tradition says he was summoned with a proposal to regulate the flooding of the Nile, realised the project was impossible, and feigned madness for years under house arrest to escape the caliph's capricious wrath. It was during this confinement — or just after — that he wrote his great work Kitab al-Manazir, the Book of Optics. The seven volumes would be the field's unchallenged reference for the next six centuries.

The problem inherited two large answers from the Greeks. Euclid and Ptolemy held that the eye was an active organ emitting rays (the extramissionist view): we see things because visual rays leaving the eye touch them. Aristotle made the air an intermediary, but vision itself remained obscure. Ibn al-Haytham rejected both. With small holes opened into darkened rooms (the camera obscura), with systematic observations on mirrors and lenses, with the after-image left in the eye by a bright object, with measurements of how light behaves at angles and intensities, he made the result clear: light comes from objects to the eye; the eye is only a receiver.

Beyond the physical claim, the book has a wider importance. Ibn al-Haytham described his method himself: anyone who wishes to speak with certainty must first state the hypothesis explicitly, then design an experiment that can test it, express the result mathematically, and accept the hypothesis only if the experiment holds. Trusting an authority — even Aristotle — is not enough. "The seeker of truth proceeds not by trust in persons but by impersonal verification." Historians of science cite this passage as the earliest clear formulation of the modern scientific method — six hundred years before Galileo, five hundred before Bacon.

Kitab al-Manazir was translated into Latin late in the 12th century as De Aspectibus. Roger Bacon, Witelo, and Kepler all read it directly; Kepler names Ibn al-Haytham in the preface of his 1604 Astronomiae Pars Optica. The principle of the camera, the lens-and-retina structure of the eye, the geometry of the rainbow, the correct reading of shadows on the lunar surface — all of these advanced along the path he opened. In 1969 NASA named a crater on the Moon after Alhazen; that naming is not an accident but a debt set down in writing.