Ibn al-Haytham

Chapter 1 · Basra to Cairo

A scholar summoned to dam the Nile

c. 965 – c. 1011 · Basra · Cairo

Abu Ali al-Hasan ibn al-Haytham was born at Basra, in what is now southern Iraq, around 965, and spent his working life in Cairo under the Fatimid caliphate. Very little of his personal life is documented; almost everything we know comes from later biographical dictionaries and from the enormous list of works attributed to him.

The episode most often told about him is the Nile. The caliph al-Hakim, hearing that this scholar claimed the river's annual flooding could be regulated, summoned him to Egypt to do it. Ibn al-Haytham travelled south, examined the ground near Aswan, and concluded that with the engineering available to him the project was impossible.

Al-Hakim was a dangerous man to disappoint. The tradition holds that Ibn al-Haytham feigned madness — a condition that placed a person outside ordinary accountability — and was accordingly confined rather than executed, remaining under house arrest until the caliph's death in 1021, whereupon he recovered his wits.

This story should be handled with care. It comes from sources written well after the events, and some historians regard it as a legend attached to a famous name. What is not in dispute is that he produced his greatest work in Cairo in the years around 1021, and that he lived by copying manuscripts of Euclid and Ptolemy for money.

Why this matters

The confinement story is the best-known thing about him and the least verifiable, which makes it a good early lesson in weighing the sources for a life.

You have the impossible commission and the years that followed. What would you ask him?

Ask Ibn al-Haytham

• “Did you really feign madness to escape al-Hakim?”
• “What did you see at Aswan that told you the dam could not be built?”
• “What was it like to earn a living copying Euclid and Ptolemy?”
• “Why did you leave Basra for Cairo at all?”
• “How much of what is written about your life would you recognise?”

Chapter 2 · Which Way Does Seeing Go?

Two arguments and one experiment

c. 1021 · Cairo

The Greeks had left the question of vision unsettled, and both sides had serious advocates. Euclid and Ptolemy held the emission theory: the eye sends out rays that reach and feel the object, which is why sight seems instantaneous and directional. Others, including the atomists, held that objects send something to the eye.

Ibn al-Haytham decided it, and his arguments are worth following because they are the kind anybody can check. If the eye emits the agent of vision, why does staring at the Sun hurt the eye? The damage comes from outside. Why can we not see in a dark room, if the eye supplies what is needed? Why, after looking at a bright light, does an after-image persist? And how could the eye possibly emit enough to reach the fixed stars the instant it opens?

His conclusion: light travels in straight lines from every point of an object, in all directions, and vision occurs when some of that light enters the eye and is ordered there. The eye is a receiver.

He then had to solve a problem this created. If every point of the eye receives light from every point of the object, why is the image not a blur? His answer — that only the rays striking the surface of the eye perpendicularly are effective — is wrong, but it is a real attempt at a real difficulty rather than a wave of the hand.

“The seeker after truth is not one who studies the writings of the ancients and puts his trust in them, but rather the one who suspects his faith in them and questions what he gathers from them.”

— Ibn al-Haytham, Doubts Concerning Ptolemy (Al-Shukuk ala Batlamyus)

Why this matters

The move from arguing about vision to testing claims about it is often cited as the earliest clear statement of experimental method.

You have the eye settled as a receiver. What would you put to him?

Ask Ibn al-Haytham

• “What made you certain the eye receives rather than emits?”
• “Why does it hurt to look at the Sun, on your account?”
• “If light comes from every point, why is what I see not a blur?”
• “How do you decide when an ancient authority must be abandoned?”
• “Where did you think Euclid and Ptolemy went wrong?”

Chapter 3 · The Darkened Chamber

An inverted world on a wall

c. 1021 · Cairo

Ibn al-Haytham darkened a room, admitted light through a small hole in one wall, and found an image of the scene outside cast on the opposite wall — upside down and reversed left to right. He described it clearly enough that his account is the first unambiguous one we have of what would later be called the *camera obscura*.

He used it as evidence, not as a curiosity. If light travelled any way but in straight lines, no such image could form; the fact that it does, and that it inverts, is a direct demonstration that rays cross at the aperture and continue straight. He also observed that light from several sources passing through the same hole does not mix or interfere on the way — each keeps its own path — which he took as further evidence of independent rectilinear rays.

He experimented with the size of the aperture and noticed the trade-off any photographer knows: a smaller hole gives a sharper but fainter image.

His wider optical work was equally systematic. He studied reflection from plane, spherical, cylindrical and conical mirrors, and posed the problem — still called Alhazen's problem — of finding the point on a spherical mirror at which a ray from a given source reflects to a given eye. He solved it geometrically. It is not solvable with ruler and compasses alone.

Why this matters

The camera obscura made an abstract claim about light visible on a wall — an early example of an experiment that shows rather than argues.

You have the hole in the wall and the upside-down world. What is your question?

Ask Ibn al-Haytham

• “Why is the image in the darkened room upside down?”
• “What happens when you make the hole smaller?”
• “How did you know two beams crossing do not disturb each other?”
• “What is the problem on the spherical mirror that carries your name?”
• “Why write optics as geometry rather than as description?”

Chapter 4 · The Law He Could Not Find

Knowing Ptolemy is wrong is not the same as being right

c. 1021 – c. 1040 · Cairo

Light bends when it passes from air into water or glass. Ptolemy had published tables of measured angles and claimed a simple proportionality between the angle of incidence and the angle of refraction.

Ibn al-Haytham measured carefully and showed that this is false: the relation is not proportional, and Ptolemy's tables look suspiciously tidy, as though the numbers had been adjusted to fit the rule. He established the qualitative facts — that light bends toward the normal entering a denser medium and away from it leaving one, that the bending increases with the angle, that reflection and refraction happen together at a surface.

And then he stopped, because he could not find the correct law. It is the relation between the *sines* of the angles, and it would not be published until Snell and Descartes, six hundred years later.

This is worth dwelling on. He had the apparatus, the measurements and the knowledge that the accepted rule was wrong, and the right answer still escaped him. It is a useful corrective to the idea that good method produces results on demand.

He also had no conception of light as a wave, no notion that it has a finite speed, and no colour theory worth the name. His achievement was to establish how the subject should be investigated, and to leave a great deal of it open.

Why this matters

Ibn al-Haytham proving the accepted law of refraction wrong without finding the right one is an honest picture of how science usually advances.

You have the failure at the centre of the great work. What would you ask?

Ask Ibn al-Haytham

• “How did you know Ptolemy's refraction tables were wrong?”
• “What stopped you from finding the true law?”
• “Is it worth publishing that a rule is wrong if you have no replacement?”
• “What did you think light actually was?”
• “Did you suspect light took time to travel?”

Chapter 5 · Alhazen

Six centuries of influence under a borrowed name

c. 1040 – 1700 · Cairo · Toledo · Oxford · Prague

Ibn al-Haytham died in Cairo around 1040. His *Kitab al-Manazir*, the Book of Optics, was translated into Latin in the late twelfth or early thirteenth century as *De aspectibus*, under the author name Alhazen.

Under that name it became the foundation of European optics. Roger Bacon and Witelo built directly on it in the thirteenth century. It was printed in 1572 in Basel. Kepler read it and worked from it, and it stands behind his correct account of the retinal image; Descartes and the seventeenth-century opticians were working in a tradition it had established.

So the influence is enormous and the name is a distortion. For centuries European readers knew a Latinised label and knew almost nothing about the person, the city, or the intellectual world the book came from. Recovering that has been substantially a twentieth- and twenty-first-century project.

There is also no likeness of him. No contemporary portrait, description or medal survives; every modern image is invention. In Incandio he is represented without a portrait for that reason, and the absence is itself a fact about how the record was kept.

What lasts is not only the optics. It is the position he stated plainly: that the person seeking truth is the one who suspects the ancients and questions what they wrote — including, he was careful to say, himself.

Why this matters

The Book of Optics governed European optics for six hundred years under a Latinised name, while its author's world was largely forgotten by the readers who depended on him.

You have six centuries of influence under the wrong name. What is your question?

Ask Ibn al-Haytham

• “How would you feel about being known in Europe as Alhazen?”
• “What did Kepler take from your book?”
• “You told readers to doubt you as well — did anyone?”
• “Does it matter that no likeness of you survives?”
• “Which of your results would you most want checked again?”

What Ibn al-Haytham changed

Optics as an experimental science begins with the Book of Optics: light travelling in straight lines, the eye as a receiver, the camera obscura, and the systematic study of mirrors are all in it. Through its Latin translation it shaped European optics from Roger Bacon to Kepler. His insistence that received authority must be tested rather than trusted is regularly cited as the earliest clear statement of scientific method.

A debate that continues

The story of feigned madness under al-Hakim rests on late sources and is doubted by some historians, and the attribution of the very large list of works bearing his name is not fully settled.

Keep exploring — ask Ibn al-Haytham

• “Which ancient authority was hardest for you to abandon?”
• “How would you design the refraction experiment again?”
• “What should a reader doubt in your own book?”

Related lives

• Euclid — Who Made Proof The Standard
• Claudius Ptolemy — Who Tabulated the Heavens
• Johannes Kepler — Discoverer of Planetary Motion
• Muhammad al-Khwarizmi — Who Named Algebra And The Algorithm

Related themes

Light and reflection · Refraction · The history of scientific method