Ibn al-Haytham's Book of Optics
Between about 1011 and 1021, the scholar Ibn al-Haytham (known in Latin as Alhazen) wrote the Kitab al-Manazir, or Book of Optics, in Cairo under the Fatimid caliphate. The seven-volume work argued that we see because light reflected from objects enters the eye, rather than because the eye sends out rays. Just as important, it set out a method of testing ideas through controlled experiment that later scholars found deeply influential.
Ibn al-Haytham was born around 965 in Basra, in present-day Iraq, and was educated in the scientific traditions of the Abbasid world. By his time, Greek works had been translated into Arabic and extensively debated. Two views of vision dominated. Euclid and Ptolemy favored an "emission" theory, in which rays travel from the eye to the object. Aristotle and others leaned toward the idea that something passes from the object to the observer, while Galen offered a detailed anatomy of the eye. Ibn al-Haytham set out to resolve these competing accounts. Later biographical tradition says he moved to Cairo and spent a period under house arrest after failing to deliver on a proposal to regulate the Nile's flooding, though the details are uncertain and come from sources written well after his lifetime.
In the Book of Optics, he argued that light radiates in straight lines from every point on a luminous or illuminated object, and that vision occurs when some of this light enters the eye. He studied reflection and refraction, examined how mirrors and lenses behave, and used a darkened room with a small aperture, the camera obscura principle, to show that light travels in straight lines. He also described the structure of the eye in detail, though he still mistakenly treated the lens as the seat of sight. Crucially, he argued that perception involves judgment and inference in the mind, not just the arrival of light, which helped him explain optical illusions and the apparent size of the Moon near the horizon.
His method was as significant as his conclusions. He insisted that claims be tested by experiments designed to confirm or refute them, and he criticized reliance on authority alone. Historians debate how far this makes him a forerunner of the modern scientific method, but his procedures were unusually systematic for the era.
The book was translated into Latin around the late twelfth or thirteenth century and shaped European thinkers such as Roger Bacon, Witelo, and later Johannes Kepler, who built on its ideas in his work on the retina and image formation. Ibn al-Haytham is believed to have died around 1040.