Abu Ali al-Hasan ibn al-Haytham (c. AD 965–c. 1040), known in Latin Europe as Alhazen, was born in Basra, in today's Iraq, and did most of his known work in Cairo. He wrote on mathematics, astronomy and optics. His Book of Optics (in Arabic, Kitab al-Manazir), written in the early 1000s, changed how people understood light and sight.
Key idea
The problem
How do we see? Some ancient thinkers, such as Euclid and Ptolemy, said the eye sends out rays that touch things, like feeling with a stick. Others, following Aristotle, said objects send something to the eye. Each side had arguments. Ibn al-Haytham asked which idea fits what we can test.
How the idea came
He pointed out that looking at the Sun hurts the eye, and a bright light leaves an afterimage. Something is coming in. And when you open your eyes at night, you see distant stars at once: could rays from the eye really reach them that fast? He then built a full theory: every point of a lit object sends light out in all directions, in straight lines, and the eye collects it.
The dark room experiment
He described setting several lamps outside a room with a small hole in its wall. Each lamp made its own spot of light on the wall inside. Cover one lamp and its spot goes dark, and only that one. So light from each lamp travels in its own straight line through the hole, and the lines cross without mixing. Later scientists called a darkened room with a hole a camera obscura, Latin for "dark room."
Why it works
Worked example: How big is the image?
- The two triangles, tree to hole and hole to wall, have the same shape.
- $\dfrac{h}{6} = \dfrac{0.5}{20}$, so $h = 6 \times 0.025 = 0.15$ m.
Answer: 0.15 m, or 15 cm, upside down.
Testing, not just arguing
What makes him stand out is his method. He tested claims with set-ups he described carefully, so others could repeat them, and he checked his ideas against measurements. He also wrote a book of Doubts Concerning Ptolemy, listing places where the great astronomer's work did not hold together. Many historians count him among the founders of experimental science; others point out that experiment had older roots too. Both agree his optics was a big step.
Story or history?
A popular story says Ibn al-Haytham pretended to be mad to escape an impossible job: the Caliph al-Hakim had asked him to control the Nile's floods, and he saw it couldn't be done. The story comes from biographers writing after his death, and historians treat it with caution.
| When | What happened |
|---|---|
| c. AD 965 | Born in Basra |
| early 1000s | Moves to Cairo under the Fatimid caliphs |
| c. 1011–1021 | Writes the Book of Optics (the dates are approximate) |
| c. 1040 | Dies in Cairo |
| c. 1200 | The Book of Optics is translated into Latin as De aspectibus |
| 1572 | Printed in Basel; read by Kepler and others |
Worked example: A long road to print
- $1572 - 1021 = 551$.
Answer: About 550 years.
What it led to
Latin scholars such as Roger Bacon and Witelo built on his optics. Kepler, around 1604, used his ideas to explain how the lens of the eye forms an upside-down picture on the retina. Leonardo da Vinci compared the eye to a dark room with a hole, and the camera obscura became a tool for artists and the ancestor of the photographic camera.
Common mistake
Build it yourself
Read more: Ibn al-Haytham, Book of Optics and Camera obscura.