Ibn al-Haytham’s Optics: how light, the eye and experiment explained sight

Today every pocket holds a camera. Inside it, a small opening and a lens take in light and form a picture. Yet the question of how we see puzzled people for thousands of years. In the 11th century, a scholar living in Cairo, Ibn al-Haytham, answered it with argument and experiment. In Europe he became known as Alhazen, and his "Book of Optics" (Kitab al-Manazir) remained the basic book on optics for centuries.

From Basra to Cairo

According to the MacTutor archive of St Andrews University, Ibn al-Haytham was born in 965, probably in Basra (in present-day Iraq), and died around 1040, probably in Cairo. He was already known as a scholar in Basra. MacTutor says he went to Egypt in connection with an engineering project to regulate the waters of the Nile. He soon realised that the project was not feasible. After that, he spent much of his life in scholarly work in Egypt.

According to MacTutor, he wrote about 92 books and treatises, of which more than 55 survive today. His most important work is considered to be the "Book of Optics".

The Book of Optics

The book has seven parts. According to MacTutor, the first deals with the process of seeing and the structure of the eye, the second with visual perception, and the third with the conditions for good eyesight and the causes of optical illusions. The fourth part deals with reflection, the bouncing of light off a mirror. The fifth contains a famous mathematical problem that is still called "Alhazen’s problem". The sixth covers errors of vision caused by mirrors, and the seventh covers refraction, the bending of light as it enters water or glass. According to MacTutor, Ibn al-Haytham saw light as a motion whose speed falls in dense matter.

The date of writing is disputed. Unesco marked 2015 as the year the book completed nearly 1,000 years, while Thomas Leisten, a researcher associated with Princeton University, says it was written in the 1030s.

How we see

In ancient times, one popular view was that a ray leaves the eye, reaches objects and so lets us see them. Another was that some influence travels from objects to the eye. According to MacTutor, Ibn al-Haytham gave the first correct explanation of vision: light strikes an object and enters the eye, and this is how we see the object.

According to Leisten, Ibn al-Haytham tested the theory of rays leaving the eye against everyday observations and systematic experiments. For example, looking at a bright light hurts the eye, the image of a light stays in the eye for a while after one looks away, and in darkness no one can see anything. If the eye itself sent out rays, seeing in the dark would be possible too.

MacTutor also notes that, because he had no concept of the lens, some of his explanations of the eye’s inner structure were not correct. This is how science moves forward, step by step.

Experiment: the test of knowledge

Ibn al-Haytham’s greatest contribution may be his method. According to MacTutor, he wrote that light would be investigated through experimental evidence rather than abstract theory. According to Leisten, the fourth and fifth parts of the book are in effect collections of experiments with which he proves his claims. According to MacTutor, in the fourth part he gave an experimental proof of the principle of reflection and described a copper instrument for measuring reflection.

Writing in Nature Middle East, a Nature Group journal, Mohammed Yahia says Ibn al-Haytham wrote his book so that the reader could repeat his experiments himself. The same article quotes his advice that a critical reader should make himself an enemy of everything he reads and test it. In today’s language, this is the scientific method: a claim, an experiment, and then a chance for others to repeat it.

The dark room and the camera

If a closed, dark room has a small hole in one wall, the scene outside appears upside down on the opposite wall. This arrangement is called the "camera obscura" in Latin, meaning dark room. According to MacTutor, Ibn al-Haytham is the first person to mention it in writing. Today’s camera also works on the same simple principle: light passes through a narrow opening and forms an image.

Latin translation and influence

According to Leisten, in the mid-12th century the book was translated into Latin as "De aspectibus", probably in Spain or Italy. The 13th-century European scholar Witelo based his book "Perspectiva" on it, and its influence reached Roger Bacon in England and, later, the German astronomer Johannes Kepler. According to the Arabic-Latin corpus of the University of Würzburg in Germany, Friedrich Risner published its Latin text in 1572 in Basel, Switzerland.

The United Nations declared 2015 the International Year of Light, and according to Nature Middle East, 1,000 years of the "Book of Optics" were also marked on the occasion. When a student in Quetta today takes a photograph with a mobile camera, or puts on glasses to read a book, he benefits from the answer to the same question that Ibn al-Haytham found by experiment a thousand years ago.