Ibn al Haytham (Alhazen) is considered the initiator of modern optics and the scientific method.

The latter is based (1) on reciprocal relation between experiment/observation and theory (expressed here in mathematical language) and (2) on primacy of verdict of the experiment

Starting from four propositions based on experiment, Ibn al-Haytham develops optics, like Euclid, who develops geometry from five axioms.

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Publication of its experimental/observational data is accompanied by that of description of different steps if measurements as well as of measuring instruments in order to allow verification of results by others. It opens the way of doing modern physics, resumed five or six centuries later in Europe.

Ibn al-Haytham deals with the vision and perception of images by the eye. This allows him to establish an objective conception of the world, treating the observed object as independent of the observing subject. The eye thus acquires its scientific status: that of first detector. Moreover, Ibn al-Haytham practices methodical doubt by subjecting not only the writings of the ancients but also his own prejudices to ruthless criticism.

Today, while priority to verdict of practice is in many respects challenged - in particular in cosmology , contemporaneity of Ibn al-Haytham is more evident than ever.

Ibn al-Haytham, known in Western Europe as Alhazen, was born in Basra (Iraq) in 965 and died in Cairo (Egypt) in 1039. There he wrote several books on various subjects (astronomy, medicine, mathematics, physics, psychology, scientific method, etc.). He is above all a physicist. From Claude Ptolemy (~90 -168) to Nicolas Copernicus (1473 - 1543), Ibn al-Haytham is the major figure of physics.

Between 1015 and 1021, he wrote his major work, “Book of Optics” [in Arabic “Kitab al-Manazir”, in Latin “De Aspectibus” or “Opticae Thesaurus: Albazeni Arabis”]. This book had an important influence on the development of optics and physics in general because it radically transformed the knowledge of light and vision and introduced the scientific method. It was not until the two treaties in optics, of Johannes Kepler (1571-1630) that a real advance was ade in this field.

” His work on optics, which includes a theory if vision and a theory of light, is considered by many to be his most important contribution, setting the scene for developments well into the seventeenth century. His contributions to geometry and number theory go well beyond the Archimedean tradition. And by promoting the use of experiments in scientific research, Ibn al-Haytham played an important part in setting the scene for modern science.” (Roshdu Rashed)

It is from the point of view of 20th-21st century physicist that I will discuss with you about Ibn al-Haytham and not a historian, because that is not my job. Historians such as Roshdi Rashed have done important researches on this subject in the last half-century.

I was interested in Ibn al-Haytham for several reasons. Two of them here. First, there is the underestimation Europe and the United states of the profoundly original contribution of the Arab world to physics between the 8th and 13th centuries. This case, fir example, of Steven Weinberg’s otherwise excellent book “To Explain the World. The Discovery if Modern Science “(2015). Next, in this 21st century, the scientific method, initiated by Ibn al-Haytham, is challenged by various brilliant theorists specialized in superstrings, cosmology or speculating on the existence of a multiverse. Arguing that it would not be possible to experimentally verify the validity of their speculative theories, they come to conclude that the logical coherence of their theories willsuffice.

I was interested in Ibn al-Haytham for several reasons. Two of them here. First, there is the underestimation Europe and the United states of the profoundly original contribution of the Arab world to physics between the 8th and 13th centuries. This case, fir example, of Steven Weinberg’s otherwise excellent book “To Explain the World. The Discovery if Modern Science “(2015). Next, in this 21st century, the scientific method, initiated by Ibn al-Haytham, is challenged by various brilliant theorists specialized in superstrings, cosmology or speculating on the existence of a multiverse. Arguing that it would not be possible to experimentally verify the validity of their speculative theories, they come to conclude that the logical coherence of their theories will

1-Optics :

Historically, optics appeared as early as Antiquity, and then was developed by Muslim scholars. Ibn al-Haytham is considered the father of modern optics. Latin translation of part of his work, Book of Optics, had a great influence on scientists in the West, particularly on Kepler.

The geometrical optics introduced by Ibn al-Haytham developed on the basis of simple observations and is based in four propositions:

rectilinear propagation in a transparent ( homogeneous and isotropic) medium;
reverse return principle, which expresses the reciprocity of light path between transmission and reception;
the law of reflection of light;
the law of refraction of light [attributed to Snell and Descartes]

Geometrical optics makes it possible to find almost all the results concerning mirrors, lenses, optical instruments, etc. Problem solving is done using geometric constructions (straight lines representing light rays, angle calculations). Geometrical optics works well as long as polarization or interference phenomena are not modeled and no dimension of the system is comparable or less than the wavelength or the light used.

Ibn al-Haytham, by multiple experiments, shows that light - a ray of light - propagates in transparent media in a recliner manner. He counters other conceptions such as a diffuse propagation of it.

“How does light travel through transparent bodies? Light passes through transparent media in a straight line only (...). We have explained this exhaustively in our Book of Optics. But let us now mention something to prove it convincingly :the fact that light travels in a straight line is clearly observed when light enters a dark room through holes. … the light path will be clearly observable thanks it the dust suspended in the air.”

Note that the discovery of the law of refraction,

Sin θ1 / Sin θ2 = constant

with θ1 and θ2 , the angle of incidence and the angle of refraction respectively, attributed to Snell and Descartes living in the 17th century, was actually made by Ibn Sahl (940-1000) in 983, as shown in 1990 by Rashed Roshdi. More than 600 years before Snell and Descartes. However Ibn al-Haytham knee the law of refraction in approximate from ( θ1/θ2=constant) only.

A characteristic of modern physics is that theoretical formulation in different fields is based on principles and laws that can be formulated is compactly that they can be printed on a t-shirt:

Geometric optics (Ibn al-Haytham), mechanics (Newton), gravitation (Newton and Einstein), thermodynamics, quantum mechanics, standard models if fundamental interactions, etc.

While Euclid bases his geometry on 5 axioms to logically deduce multiple theorems, Ibn al-Haytham, on the basis of 4 propositions, derives multiple results from geometrical optics which he tests experimentally. It was a novelty in the natural sciences. Chemistry or biology relied in too few experimental facts at the time it achieves such a level if logical coherence reflecting reality.

Among his many contributions, he was the first to explain why the sun and moon seem larger when they are near horizon he established that moonlight comes from the sun; he understood that the phenomenon of dawn and dusk (light at sunrise and sunset without seeing the sun) is due to a phenomenon of refraction in the atmosphere; etc.

To Ibn al-Haytham applies what Hans Bethe (Nobel Prizes on Physics 1967) told me 50 years ago: “In research, it is not enough to have sufficient intellectual qualities, you must also have the intelligence to choose a subject an area where you can make an important contribution, taking into account your own limitations.” one thousand year ago, geometrical optics was such a field, unlike for example, Ptolemy’s world model.

Thank you for your reading, and I will wait for your reply.

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