Who was Ibn al-Haytham? The scholar who solved the secret of seeing through experiment

How do you see an apple on a table? Today we know the answer: light reflects off the apple and enters your eye, and your brain turns that signal into an image. But the answer has not always been so obvious. Some of the most respected thinkers of the ancient world believed that invisible rays came out of our eyes and that these rays made sight possible by “touching” objects. The person who changed this debate for good was a scholar who lived about a thousand years ago: Ibn al-Haytham.

From Basra to Cairo

This scholar, whose full name was Abu Ali al-Hasan ibn al-Haytham, was born around 965 in Basra, in present-day Iraq, and died around 1040 in Cairo. In Europe he is known by his Latin name, Alhazen. He is reported to have written more than a hundred works on mathematics, astronomy, optics and philosophy, but only some of them have survived.

The most famous story about his life goes like this: after Ibn al-Haytham claimed he could regulate the floods of the Nile, he was summoned to Egypt by the Fatimid caliph al-Hakim. But when he travelled south along the river and examined the conditions, he realised the plan could not be carried out. It is said that, fearing the caliph’s anger, he pretended to be mad and lived under watch in his house until the caliph’s death. This story comes from biographers who lived after him, and nobody knows how accurate its details are. Even so, many historians think he worked on his most important book during this period.

Two old ideas about seeing

Before Ibn al-Haytham there were two main views on how we see:

• The emission view: mathematicians such as Euclid and Ptolemy argued that rays leaving the eye form a cone and that sight happens when they reach objects. This view could be explained well with geometry, but it was physically odd: how could rays reach distant stars the moment we open our eyes?
• The intromission view: thinkers such as Aristotle suggested that something coming from objects reaches the eye. But they could not explain mathematically how this happened.

Ibn al-Haytham’s great contribution was to bring these two approaches together: he argued that seeing happens when light from objects enters the eye, and he studied the path of this light with geometry, as straight lines.

Kitab al-Manazir: the Book of Optics

Ibn al-Haytham’s most important work is the seven-volume Kitab al-Manazir (Book of Optics), thought to have been written between about 1011 and 1021. In it he examined in detail the reflection and refraction of light, the structure of the eye and visual illusions.

Here is a simple but powerful example of his reasoning: when we look at a very bright light source, such as the Sun, our eyes hurt, and we even keep seeing an afterimage for a while after closing them. This shows that something coming from outside acts on the eye. If rays came out of the eye, why would looking at a bright object hurt?

The dark-room experiment

One of Ibn al-Haytham’s most famous experiments involves the dark room (known by its Latin name, camera obscura). It was already known that if a small hole is made in the wall of a windowless room, the scene outside falls upside down on the opposite wall. Ibn al-Haytham studied this effect systematically.

In one of the experiments described, he places several candles side by side outside the dark room. Inside the room he sees that each candle makes its own separate spot of light. When he covers one candle, only the spot of light that matches it disappears. Two important conclusions follow:

1. Light travels in straight lines.
2. Rays from different sources do not mix with each other, even when they cross at the hole.

The working principle of cameras today, and of our own eyes, rests on this basic idea. The pupil is like a small hole, and the image that falls on the retina is actually upside down; the brain turns it the right way up.

The giant Moon on the horizon and visual illusions

Ibn al-Haytham was interested not only in the physics of light but also in how the brain interprets what we see. The best example is the Moon illusion. When the Moon is near the horizon it looks much bigger than when it is high in the sky. Yet the angle the Moon takes up in our field of view is almost the same in both cases; you can easily check this with a camera.

Ibn al-Haytham argued that the difference comes not from the eye itself but from perception. When we look at the Moon near the horizon we see the hills, buildings and land in between, so the mind takes the Moon to be further away. And an object that takes up the same angle but that we think is further away is perceived as bigger. The full explanation of the Moon illusion is still debated by researchers today; but linking the effect to a psychological process of judgement was a very advanced idea for his time.

In his book he also tackled a hard geometry problem about finding the point of reflection on a spherical mirror. In Europe this problem was long known by his Latin name, as “Alhazen’s problem”.

The experimental method: myth or fact?

Ibn al-Haytham is often called “the first scientist” or “the inventor of the scientific method”. These phrases express a truth in an exaggerated way.

What is true: Ibn al-Haytham applied, very systematically, the practice of testing claims with carefully designed experiments and expressing the results in mathematics. In his works he gave a central place to the Arabic concept of “i’tibar”, the idea of testing by experiment. He also did not hesitate to question earlier authorities, even Ptolemy, whom he respected, and he wrote a book criticising the contradictions in Ptolemy’s works.

What is exaggerated: science was not “invented” by a single person. In ancient times thinkers such as Archimedes and Ptolemy also made measurements and experiments; after Ibn al-Haytham, Roger Bacon, Kepler, Galileo and others in Europe developed this tradition further. The scientific method took shape over centuries with contributions from many cultures. Ibn al-Haytham is one of the strongest links in this long chain.

Influence reaching Europe

Kitab al-Manazir was translated into Latin around 1200 and became known in Europe under the titles De aspectibus or Perspectiva. The 13th-century thinkers Roger Bacon and Witelo used his ideas. At the start of the 17th century, Johannes Kepler built on this tradition when he explained that the eye, like a lens, forms an image on the retina.

The thousandth anniversary of Ibn al-Haytham’s work was also marked in 2015 as part of the International Year of Light declared by UNESCO.

Connection to today

The lenses in glasses, phone cameras, microscopes and telescopes exist because we understand how light reflects and refracts. More importantly, Ibn al-Haytham’s approach offers every student a valuable habit: test a claim with evidence, no matter who makes it. When you design an experiment in a science lesson and follow the steps “change the variable, observe the result, repeat”, you are continuing a thousand-year-old tradition.

Summary

• Ibn al-Haytham (about 965-1040) was a mathematician and physicist born in Basra who worked in Cairo.
• He argued that seeing happens when light from objects enters the eye, and he explained this with geometry.
• With dark-room experiments he showed that light travels in straight lines.
• Kitab al-Manazir was translated into Latin and had a deep influence on the study of optics in Europe.
• He is not “the sole inventor of the scientific method”, but he is one of the most important figures in the development of the experimental method.