Ibn Al Haytham corrected Ptolemy’s theory of vision and paved the way for Johannes Kepler to develop it further Image Credit: Supplied

Better known as Alhazen (Alhacen) in the West, Ibn Al Haytham’s contributions to the principles of optics and the use of scientific experiments allowed for significant progress in the fields of astronomy and ophthalmology. He was one of the great mathematicians who left their marks on humanity. He was called the “Physicist” in medieval Europe and nicknamed Ptolemaeus Secundus (Ptolemy the Second), according to the historian Henry Corbin.

Controversial beginnings

Abu Ali Al Hassan Ibn Al Hassan Ibn Al Haytham was born in Basra, Iraq, on July 1, 965. From Basra, the young man moved to Baghdad, the cultural capital of the relatively emancipated Abbasid Caliphate, where he received one of the best educations available at the time. How he ended up in Cairo, Egypt, is a mystery but presumably he was recruited for his expertise to regulate river floods. One of the numerous conflicting stories about him is the claim that there were two different Ibn Al Haythams — Al Hassan Ibn Al Hassan, the mathematician who wrote on optics, and Mohammad Ibn Al Hassan, the astronomer-philosopher who wrote an autobiography in 1027 along with a variety of studies. Even more intriguing was that Ibn Al Haytham became caught up in palace intrigues after, allegedly, he was tasked to regulate the flow of the Nile precisely to prevent recurrent floods.

According to one version of his biography, when the third ruler of the Fatimid dynasty, the Caliph Abu Ali Mansour Tariqul Hakim, or Al Hakim Bin Amr Allah heard of Ibn Al Haytham’s boasts, he ordered the scientist to carry out this operation, which proved to be impossible.

When he realised that the task required a large dam and was technically difficult, he feigned madness to avoid the AL Hakim’s opprobrium, although failure to find a solution still earned him house arrest starting in 1011 until the caliph’s death in 1021. Ibn Al Haytham put this time to good use and produced some of his best scientific ideas, including his opus, “The Book of Optics”.

After Al Hakim’s rule, and on account of his works, he was able to prove that he was not mad. He wrote several new treatises on physics, astronomy and mathematics, travelled to Spain, which was then under Islamic rule, and conducted experiments with his counterparts at leading universities. He died in Cairo, Egypt, on March 6, 1040.

The Book of Optics

Ibn Al Haytham’s most important work is “Kitab Al Manazir” (The Book of Optics), a seven-volume treatise written between 1011 and 1021, and translated into Latin at the end of the 12th century. Although the Basrawih relied on Ptolemy’s “Optics”, it was his own opus that contained the correct model of vision. He explained the passive reception by the eyes of light rays that are reflected from objects, not an active emanation of light rays from the eyes, which was what Ptolemy postulated. The “Kitab Al Manazir” was based on experiments and backed by mathematical reasoning, even if most of the theory was validated rather than “discovered”, before being finally proven several centuries later when various machines were invented to depict light’s direction. What stood out in the 11th-century work, nevertheless, was its “complete formulation of the laws of reflection and a detailed investigation of refraction, including experiments involving angles of incidence and deviation”. The “Kitab” contained Alhazen’s problem — “to determine the point of reflection from a plane or curved surface, given the centre of the eye and the observed point — which is stated and solved by means of conic sections”. Indeed, refraction was correctly “explained by light’s moving slower in denser mediums”, in his “Al Shukuk ‘ala Batlamyus” (Doubts about Ptolemy), where the Iraqi criticised Ptolemy’s “Optics” (as well as the latter’s planetary hypotheses).

It must be emphasised that Ptolemy’s theory of vision had prevailed until that time — that one could see because the eyes themselves emitted rays of light. Aristotle advanced a different theory, namely that physical forms entered the eye from an object, and were thus “seen”. By combining parts of the mathematical arguments on rays advanced by Euclid with the medical tradition of the Roman physician Galen, and the intromission theories of Aristotle, Ibn Al Haytham theorised that for each point on a seen object, there was a corresponding point on the eye. Remarkably, he further avowed that the eye only perceived perpendicular rays from an object — only the ray which reached a point on the eye directly without being refracted by any other part of the eye.

Consequently, he set out the initial arguments that confirmed the existence of an optical nerve, which interpreted how objects sending many rays to the eye could be seen even if only the perpendicular ray mattered. It was the one-to-one correspondence that mattered as other rays would, in turn, be refracted through the eye and perceived as if they were perpendicular. Although it fell on Johannes Kepler to advance the retinal image theory, Ibn Al Haytham may be said to have placed the great German scientist on the right track. Indeed, while Kepler, one of the most important figures of the 17th-century scientific revolution, was best known for his laws of planetary motion, contemporary neuroscientists credit him for his study of optics. Indeed, he was the first to recognise that images are projected, inverted and reversed by the eye lens on to the retina, and though he suggested that the image was corrected “in the hollows of the brain” due to the “activity of the soul”, it was safe to say that he built his experiments on Ibn Al Haytham’s pioneering work. Unlike Kepler, Ibn Al Haytham avoided the image inversion in his theory of vision, even if he described how he thought the eye was anatomically constructed and how this anatomy behaved functionally as an optical system. Had he understood pinhole projection instead of maintaining that the rays that fell perpendicularly on the lens passed into the optic nerve, then he could have solved the problem long before Kepler. Ibn Al Haytham had followed Galen in believing that the lens was the receptive organ of sight, although he did not exclude a role for the retina.

Finally, it is also critical to note that Ibn Al Haytham showed through experiments that light travels in a straight line, and carried out various experiments with lenses and mirrors to assess refraction and reflection. These experiments considered separately the vertical and horizontal components of reflected and refracted light rays, which was an important step in understanding optics geometrically.

As if revolutionising the way humanity understands light — which led to the development of a slew of scientific inventions we take for granted — was not enough, Ibn Al Haytham pioneered several other discoveries that are worthy of attention. In the 1020s and 1030s, he wrote numerous books on astronomy, caught the mistakes of the Ptolemaic model as to how the stars and planets moved, and provided a more realistic view of the way the universe worked. Although he postulated that the Earth was a sphere, he stuck to the ancient Greek idea that it was the centre of the universe, probably out of religious fear. Such concerns notwithstanding, he completely refuted astrology as a scientific subject, as he affirmed that scientific ideas required proof. The ideas of astrology were not rooted in any type of science, he maintained, but in the thoughts and feelings of astrologers. He dismissed astrology as a serious field of study since it contradicted one of the main ideas of Islam — that God is the cause of all things, not celestial bodies.

A worthy legacy

Ibn Al Haytham’s numerous advances in optics, physical science, and the scientific method, which were all based on actual experiments, influenced the work of the English Franciscan friar, Roger Bacon, who cited him in his study of optical systems using mirrors. The Iraqi scientist’s most useful observation was to correctly evaluate the ratio between the angle of incidence (the angle between a ray on a surface and the line perpendicular to the surface) and its refraction, which, he theorised, does not remain constant. It is this theorem that led to the discovery that a lens can have a magnifying power depending on a ray’s angle. In addition to Bacon, Ibn Rushd [Averroes] also used Ibn Haytham’s writings on optics, as did the great Persian scientist Kamal Al Deen Al Farisi whose own “Kitab Tanqih Al Manazir” was a result of the refinement of the original work.

In addition to his unique studies of optics, Ibn Haytham’s legacy was also evident in astronomy: There is the Alhazen Crater on the Moon as well as the Alhazen Asteroid (previously identified as 59239).

Last but not least, he had a great influence on Isaac Newton, who was aware of Ibn Al Haytham’s numerous studies of calculus, which led to the engineering formulae and methods used ever since. In fact, Newton’s Third Law of Motion (“for every action there is an equal and opposite reaction”) was based on Ibn Al Haytham’s postulate on the movement of bodies and the attraction between two bodies — gravity. It may thus be safe to conclude that it was not the legendary apple that fell from the tree that told Newton about gravity but the books of Ibn Al Haytham.

List of works

Ibn Al Haytham’s “Kitab Al Manazir” (The Book of Optics) was printed by the German mathematician Friedrich Risner (1533-1580), who held the first chair of mathematics at the Paris Collège Royale of France. His Opticae thesaurus: “Alhazeni Arabis libri septem, nuncprimum editi; Eiusdem liber De Crepusculis et nubium ascensionibus, Item Vitellonis Thuringopoloni libri X”, assembled Ibn Al Haytham’s works with those of Erasmus, another optics pioneer. Scientists such as Kepler, Huygens and Descartes, among others, studied Ibn Al Haytham meticulously in the Risner translation. See also Mark A. Smith, “Alhacen’s Theory of Visual Perception: A Critical Edition”, with English translation and commentary, of the first three books of Alhacen’s “De aspectibus”, the medieval Latin version of Ibn Al Haytham’s “Kitab Al Manazir”, Philadelphia: “Transactions of the American Philosophical Society”, 91-4, 91-5, & Diane Publishing, 2001.

Ibn Al Haytham’s most famous work on astronomy is “Hay’at Al ‘Alam” (On the Configuration of the World), in which he presents a non-technical description of how the abstract mathematical models of Ptolemy’s “Almagest” can be understood according to the natural philosophy of his time. In this work, the Iraqi presented a detailed description of the physical structure of the Earth, postulating that it was “round sphere whose centre is the centre of the world”. He maintained, nevertheless, that the Earth was stationary and always at rest. While this early work implicitly accepted Ptolemy’s models, a later work, “Al Shukuk ‘ala Batlamyus” (Doubts about Ptolemy), was critical of the “Almagest”.

Selected readings

1) A good general survey of Ibn Al Haytham’s work is Abdul Hamid Ibrahim Sabra, “Ibn al-Haytham”, in “Dictionary of Scientific Biography 6”, 1972, pp. 189–210.

2) For the text of nine of Ibn Al Haytham’s mathematical works, with translation and commentary, as well as the theory of the two Ibn Al Haythams, see vol 2 of Roshdi Rashed, “Les Mathématiques infinitésimales du IXe au XIe siécle”, London: Al-Furqan Islamic Heritage Foundation, 1996.

3) For a different view of this theory and for a continuation of the author’s study of Ibn Al Haytham, see Abdul Hamid Ibrahim Sabra, “One Ibn al-Haytham or Two? An Exercise in Reading the Bio-Bibliographical Sources”, “Zeitschrift für Geschichte der Arabisch-Islamischen Wissenschaften 12”, 1998, pp 1–50.

4) “Ibn al-Haytham”, “The Columbia Encyclopedia”, 6th ed, 2012, at http://www.encyclopedia.com/doc/1E1-IbnalHay.html.

5) Bradley Steffens, “Ibn al-Haytham: First Scientist”, Greensboro, NC: Morgan Reynolds Publishers, 2007.

Dr Joseph A. Kéchichian is an author, most recently of, “Legal and Political Reforms in Sa‘udi Arabia” (London: Routledge, 2013).

This article is the fourteenth of a series on Muslim thinkers who greatly influenced Arab societies across the centuries.