Ibn al-Haytham's Book of Optics (1011-1021) and Vision

Ibn al-Haytham’s Book of Optics, written in Cairo between about 1011 and 1021, is one of the most important works in the history of science. In seven books it argued that vision happens when light from objects enters the eye, and it tested its claims with careful experiments and geometry. Translated into Latin around 1200, it shaped European optics until the time of Kepler.

Background: what the ancients thought about sight

Greek thinkers disagreed about how vision works. Euclid and Ptolemy taught a form of extramission: the eye sends out visual rays that travel in straight lines and “touch” objects. This allowed precise geometry, but it raised obvious problems, such as how rays from the eye could instantly reach the stars.

Other ancient writers, including the atomists and Aristotle, held that something passes from the object to the eye instead, but they had no mathematical account of how that produced a clear image. Physicians following Galen described the anatomy of the eye in detail. Arabic scholars inherited all these traditions through translations made in centres such as the House of Wisdom in Baghdad, and al-Kindi in the ninth century wrote an influential defence of visual rays.

Ibn al-Haytham in Cairo

Abu Ali al-Hasan ibn al-Haytham was born in Basra around 965. He moved to Egypt, then ruled by the Fatimid caliphs from their new capital, described in the founding of Cairo. He spent most of the rest of his life there, earning a living partly by copying scientific texts, and died around 1040.

Later biographers tell a famous story. Ibn al-Haytham is said to have claimed that the flow of the Nile could be regulated, and the caliph al-Hakim sent him south to try. Near Aswan he realised the project was impossible and, fearing the caliph’s anger, pretended to be mad and was confined to his house until al-Hakim’s death in 1021. According to this tradition he wrote the Book of Optics during that confinement. The story appears only in sources written long after his death and cannot be confirmed.

Other works

The Book of Optics was only one of Ibn al-Haytham’s many writings. Medieval lists credit him with dozens of treatises, perhaps more than 90, though many are lost. They include works on geometry, on the conic sections, on the shape of the universe and on the motion of the planets, as well as shorter studies of the rainbow, the halo, burning mirrors and the light of the Moon.

His treatise On the Configuration of the World, describing the heavens as physical spheres, was translated into Latin, Hebrew and Castilian. In mathematics he worked on problems that later led towards calculus, such as finding the volume of a solid formed by rotating a curve. This breadth helps explain why he approached optics as a mathematician as well as a natural philosopher.

What the Book of Optics says

The Kitab al-Manazir is a long, systematic treatise in seven books. Books I to III deal with direct vision: the nature of light and colour, the structure of the eye and the way the mind perceives what the eye receives. Ibn al-Haytham argued that every point of a lit or coloured surface sends light in straight lines in all directions. Only the rays that strike the eye perpendicularly enter without being bent, and together they form an ordered image corresponding point by point to the object.

This combination was his great achievement. He kept the geometry of the old visual-ray theory but reversed the direction of travel, making light, not the eye, the active agent. He also joined physics, mathematics and anatomy into a single explanation, rather than treating them as separate subjects.

Books IV to VI deal with reflection, including mirrors of different shapes and the errors of vision they cause. In Book V he tackled a hard geometrical question: finding the point on a curved mirror at which light from a given source is reflected to a given eye. This became known in Europe as “Alhazen’s problem” and challenged mathematicians for centuries. Book VII covers refraction, the bending of light as it passes between air, water and glass.

Throughout the work Ibn al-Haytham discussed perception as well as physics. He explained that judging size, distance and shape involves inference by the mind, and he offered a psychological explanation of why the Moon appears larger near the horizon.

Experiment and proof

Ibn al-Haytham repeatedly used the Arabic word i’tibar, meaning test or verification by experiment. He described apparatus with tubes, screens and apertures to show that light travels in straight lines. In a darkened room with a small opening, he placed several candles outside and showed that each produced its own separate spot of light inside, and that covering a candle removed its spot. This demonstrated that light from different sources passes through the same opening without mixing.

He also wrote a critical work, Doubts Concerning Ptolemy, in which he pointed out contradictions in the most respected astronomer of antiquity. His attitude was that a seeker of truth should not simply trust earlier authorities but examine their arguments and test them against evidence.

Influence in the Islamic world and Europe

The original Arabic text survives in only a handful of manuscripts, several of them held in libraries in Istanbul. Critical editions of the Arabic text began to appear only in the 1980s, and the scholar A. I. Sabra produced the first English translation of Books I to III in 1989.

Around 1300 the Persian scholar Kamal al-Din al-Farisi wrote a major commentary on the Book of Optics. Building on it, he gave an explanation of the rainbow based on light refracting and reflecting inside raindrops, using experiments with a glass sphere filled with water.

In Latin Europe the work was translated around 1200, under titles such as De aspectibus. The English Franciscan Roger Bacon, the Archbishop of Canterbury John Pecham and the Polish scholar Witelo built their own works on optics, known as “perspectiva”, largely on Alhazen. In 1572 Friedrich Risner published the Latin text in Basel, together with Witelo’s optics, as the Opticae thesaurus.

Johannes Kepler wrote his own major work on optics in 1604 as a supplement to Witelo, and in it he solved the problem of how the lens of the eye focuses an image on the retina. Kepler’s theory went beyond Ibn al-Haytham’s, but it grew directly out of the tradition he had founded.

Myths and facts

“He invented the camera obscura.” The pinhole effect was known in antiquity. Ibn al-Haytham’s contribution was to study it experimentally and explain it.

“He invented the scientific method.” He was a pioneer of controlled experiment and mathematical proof, but the scientific method developed over centuries through many contributors.

“The Book of Optics was forgotten until modern times.” It was copied and commented on in the Islamic world and studied in European universities from the thirteenth century onwards.

Timeline

• c. 965: Ibn al-Haytham is born in Basra.
• 996: Al-Hakim becomes Fatimid caliph in Cairo.
• c. 1011–1021: The Book of Optics is written in Cairo.
• 1021: Death or disappearance of al-Hakim.
• c. 1040: Ibn al-Haytham dies in Cairo.
• c. 1200: Latin translation of the Book of Optics.
• 1572: Friedrich Risner prints the Opticae thesaurus in Basel.
• 1604: Kepler publishes his optics, building on the Alhazen–Witelo tradition.

Why it matters

The Book of Optics changed the question of how we see into a problem that could be solved with geometry, anatomy and experiment. Its approach influenced scientists in both the Islamic world and Europe for six centuries, and UNESCO’s International Year of Light in 2015 marked a thousand years of its legacy.