A052 - Ibn al-Haytham

 Ibn al-Haytham

Ibn al-Haytham (Abu ‘Ali al-Hasan ibn al-Haytham) (Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham) (Alhazen) (Avennathan) (965 in Basra - c. 1039 in Cairo).  Arab mathematician known in the West as Alhazen or Avennathan.   He is considered to be Islam’s greatest scientist who devoted his life to physics, astronomy, mathematics, and medicine.  His treatise Optics, in which he deftly used experiments and advanced mathematics to understand the action of light, exerted a profound influence on many European natural philosophers.  In addition to his Latinized names of Alhazen and Avennathan, Ibn al-Haytham is sometimes called al-Basri.  He is also nicknamed Ptolemaeus Secundus ("Ptolemy the Second") or simply "The Physicist" in medieval Europe.

Abu ‘Ali al-Hasan ibn al-Haytham (commonly known as Alhazen, the Latinized form of his first name, al-Hasan) was born in Basra (Iraq) in 965.  He was given a traditional Muslim education, but at an early age he became perplexed by the variety of religious beliefs and sects, because he was convinced of the unity of truth.  When he was older, he concluded that truth could be attained only in doctrines whose matter was sensible and whose form was rational.  He found such doctrines in the writings of Aristotle and in natural philosophy and mathematics. 

By devoting himself completely to learning, Alhazen achieved fame as a scholar and was given a political post at Basra.  In an attempt to obtain a better position, he claimed that he could construct a machine to regulate the flooding of the Nile.  The Fatimid caliph al-Hakim, wishing to use this sage’s expertise, persuaded him to move to Cairo.  Alhazen, to fulfill his boast, was trapped into heading an engineering mission to Egypt’s southern border.  On his way to Aswan, he began to have doubts about his plan, for he observed excellently designed and perfectly constructed buildings along the Nile, and he realized that his scheme, if it were possible, would have already been carried out by the creators of these impressive structures.  His misgivings were confirmed when he discovered that the cataracts south of Aswan made flood control impossible.  Convinced of the impracticability of his plan, and fearing the wrath of the eccentric and volatile caliph, Alhazen pretended to be mentally deranged.  Upon his return to Cairo, he was confined to his house until al-Hakim’s death in 1021. 

Alhazen then took up residence in a small domed shrine near the Azhar mosque.  Having been given back his previously sequestered property, he resumed his activities as a writer and teacher.  He may have earned his living by copying mathematical works, including Euclid’s Stoicheia (c. fourth century B.C.T.; Elements) and Mathematike suntaxis (c.150; Almagest), and may also have traveled and had contact with other scholars.

The scope of Alhazen’s work is impressive.  He wrote studies on mathematics, physics, astronomy, and medicine, as well as commentaries on the writings of Aristotle and Galen.  He was an exact observer, a skilled experimenter, and an insightful theoretician.  He put these abilities to excellent use in the field of optics.  He has been called the most important figure in optics between antiquity and the seventeenth century.  Within optics itself, the range of his interests was wide. He discussed theories of light and vision, the anatomy and diseases of the eye, reflection and refraction, the rainbow, lenses, spherical and parabolic mirrors, and the pinhole camera (camera obscura).

Alhazen’s most important work was Kitab al-Manazir, commonly known as Optics.  Not published until 1572, and only appearing in the West in the Latin translation Opticae thesaurus Alhazeni libri vii, it attempted to clarify the subject by inquiring into its principles.  He rejected Euclid’s and Ptolemy’s doctrine of visual rays (the extramission theory, which regarded vision as analogous to the sense of touch).  For example, Ptolemy attributed sight to the action of visual rays issuing conically from the observer’s eye and being reflected from various objects.  Alhazen also disagreed with past versions of the intromission theory, which treated the visible object as a source from which forms (simulacra) issued.  The atomists, for example, held that objects shed sets of atoms as a snake sheds its skin; when this set enters the eye, vision occurs.  In another version of the intromission theory, Aristotle treated the visible object as a modifier of the medium between the object and the eye.  Alhazen found the atomistic theory unconvincing because it could not explain how the image of a large mountain could enter the small pupil of the eye.  He did not like the Aristotelian theory because it could not explain how the eye could distinguish individual parts of the seen world, since objects altered the entire intervening medium.  Alhazen, in his version of the intromission theory, treated the visible object as a collection of small areas, each of which sends forth its own ray.  He believed that vision takes place through light rays reflected from every point on an object’s surface converging toward an apex in the eye.

According to Alhazen, light is an essential form in self-luminous bodies, such as the sun, and an accidental form in bodies that derive their luminosity from outside sources.  Accidental light, such as the moon, is weaker than essential light, but both forms are emitted by their respective sources in exactly the same way: noninstantaneously, from every point on the source, in all directions, and along straight lines.  To establish rectilinear propagation for essential, accidental, reflected, and refracted radiation, Alhazen performed many experiments with dark chambers, pinhole cameras, sighting tubes, and strings.

In the first book of Optics, Alhazen describes the anatomy of the eye.  His description is not original, being based largely on the work of Galen, but he modifies traditional ocular geometry to suit his own explanation of vision.  For example, he claims that sight occurs in the eye by means of the glacial humor (what would be called the crystalline lens), because when this humor is injured, vision is destroyed.  He also uses such observations as eye pain while gazing on intense light and afterimages from strongly illuminated objects to argue against the visual-ray theory, because these observations show that light is coming to the eye from the object.  With this picture of intromission established, Alhazen faces the problem of explaining how replicas as big as a mountain can pass through the tiny pupil into the eye.

He begins the solution of this problem by recognizing that every point in the eye receives a ray from every point in the visual field.  The difficulty with this punctiform analysis is that, if each point on the object sends light and color in every direction to each point of the eye, then all this radiation would arrive at the eye in total confusion.  For example, colors would arrive mixed.  Simply put, the problem is a superfluity of rays.  To explain vision, each point of the surface of the glacial humor needs to receive a ray from only one point in the visual field.  In short, it is necessary to establish a one-to-one correspondence between points in the visual field and points in the eye.

To fulfill this goal, Alhazen notices that only one ray from each point in the visual field falls perpendicularly on the convex surface of the eye.  He then proposes that all other rays, those falling at oblique angles to the eye’s surface, are refracted and so weakened that they are incapable of affecting visual power.  Alhazen even performed an experiment to show that perpendicular rays are strong and oblique rays weak. He shot a metal sphere against a dish both perpendicularly and obliquely.  The perpendicular shot fractured the plate, whereas the oblique shot bounced off harmlessly.  Thus, in his theory, the cone of perpendicular rays coming into the eye accounts for the perception of the visible object’s shape and the laws of perspective.

Book 2 of Optics contains Alhazen’s theory of cognition based on visual perception, and book 3 deals with binocular vision and visual errors.  Catoptrics (the theory of reflected light) is the subject of book 4.  Alhazen here formulates the laws of reflection. Incident and reflected rays are in the same plane, and incident and reflected angles are equal.  The equality of the angles of incidence and reflection allows Alhazen to explain the formation of an image in a plane mirror.  As throughout Optics,  Alhazen uses experiments to help establish his contentions.  For example, by throwing an iron sphere against a metal mirror at an oblique angle, he found that the incident and reflected movements of the sphere were symmetrical.  The reflected movement of the iron sphere, because of its heaviness, did not continue in a straight line, as the light ray does, but Alhazen did not contend that the iron sphere is an exact duplicate of the light ray.

Alhazen’s investigation of reflection continues in books 5 and 6 of Optics.  Book 5 contains the famous “Problem of Alhazen”: For any two points opposite a spherical reflecting surface, either convex or concave, find the point or points on the surface at which the light from one of the two points will be reflected to the other.  Today it is known that the algebraic solution of this problem leads to an equation of the fourth degree, but Alhazen solved it geometrically by the intersection of a circle and a hyperbola.

Book 7, which concludes Optics, is devoted to dioptrics (the theory of refraction).  Although Alhazen did not discover the mathematical relationship between the angles of incidence and refraction, his treatment of the phenomenon was the most extensive and enlightening before that of Rene Descartes.  As with reflection, Alhazen explores refraction through a mechanical analogy.  Light, he says, moves with great speed in a transparent medium such as air and with slower speed in a dense body such as glass or water.  The slower speed of the light ray in the denser medium is the result of the greater resistance it encounters, but this resistance is not strong enough to hinder its movement completely.  Since the refracted light ray is not strong enough to maintain its original direction in the denser medium, it moves in another direction along which its passage will be easier (that is, it turns toward the normal).  This idea of the easier and quicker path was the basis of Alhazen’s explanation of refraction, and it is a forerunner of the principle of least time associated with the name of Pierre de Fermat.

Optics was Alhazen’s most significant work and by far his best known, but he also wrote more modest treatises in which he discussed the rainbow, shadows, camera obscura, and Ptolemy’s optics as well as spheroidal and paraboloidal burning mirrors.  The ancient Greeks had a good understanding of plane mirrors, but Alhazen developed an exhaustive geometrical analysis of the more difficult problem of the formation of images in spheroidal and paraboloidal mirrors.

Although Alhazen’s achievements in astronomy do not equal those in optics, his extant works reveal his mastery of the techniques of Ptolemaic astronomy.  These works are mostly short tracts on minor problems, for example, sundials, moonlight, eclipses, parallax, and determining the gibla (the direction to be faced in prayer).  In another treatise, he was able to explain the apparent increase in size of heavenly bodies near the horizon, and he also estimated the thickness of the atmosphere.

His best astronomical work, and the only one known to the medieval West, was Hay’at al-‘alan (tenth or eleventh century; on the configuration of the world).  This treatise grew out of Alhazen’s desire that the astronomical system correspond to the true movements of actual heavenly bodies.  He therefore attacked Ptolemy’s system, in which the motions of heavenly bodies were explained in terms of imaginary points moving on imaginary circles.  In his work, Alhazen tried to discover the physical reality underlying Ptolemy’s abstract astronomical system.  He accomplished this task by viewing the heavens as a series of concentric spherical shells whose rotations were interconnected.  Alhazen’s system accounted for the apparent motions of the heavenly bodies in a clear and untechnical way, which accounts for the book’s popularity in the Middle Ages.

Alhazen’s fame as a mathematician has largely depended on his geometrical solutions of various optical problems, but more than twenty strictly mathematical treatises have survived.  Some of these deal with geometrical problems arising from his studies of Euclid’s Elements, whereas others deal with quadrature problems, that is, constructing squares equal in area to various plane figures.  He also wrote a work on lunes (figures contained between the arcs of two circles) and on the properties of conic sections.  Although he was not successful with every problem, his performance, which exhibited his masterful command of higher mathematics, has rightly won for him the admiration of later mathematicians.

For most scientific historians, Alhazen was the greatest Muslim scientist, and Optics was the most important work in the field from Ptolemy’s time to Johannes Kepler’s.  Alhazen extricated himself from the limitations of such earlier theories as the atomistic, Aristotelian, and Ptolemaic and integrated what he knew about medicine, physics, and mathematics into a single comprehensive theory of light and vision.  Although his theory contained ideas from older theories, he combined these ideas with his new insights into a fresh creation, which became the source of a new optical tradition.

Alhazen's optical theories had some influence on Islamic scientists, but their main impact was on the West.  Optics was translated from Arabic into Latin at the end of the twelfth century.  It was widely studied, and in the thirteenth century, Witelo (also known as Vitellio) made liberal use of Alhazen’s text in writing his comprehensive book on optics.  Roger Bacon, John Peckham, and Giambattista della Porta are only some of the many thinkers who were influenced by Alhazen’s work.  Indeed, it was not until Kepler, six centuries later, that work on optics progressed beyond the point to which Alhazen’s ideas had taken the subject matter.  Indeed, it would not be going too far to say that Alhazen’s optical theories defined the scope and goals of the field from his day to ours.

Al-Haitham was one of the most eminent physicists, whose contributions to optics and the scientific methods are outstanding.  Ibn al-Haitham was born in 965 in Basra (in present day Iraq), and received his education in Basra and Baghdad.  He traveled to Egypt and Spain.  He spent most of his life in Spain, where he conducted research in optics, mathematics, physics, medicine and development of scientific methods.

Al-Haitham conducted experiments on the propagation

of light and colors, optic illusions and reflections.  He examined the refraction of light rays through transparent medium (air, water) and discovered the laws of refraction.  He also carried out the first experiments on the dispersion of light into its constituent colors.  In detailing his experiment with spherical segments (glass vessels filled with water) , he came very close to discovering the theory of magnifying lenses which was developed in Italy three centuries later.  It took another three centuries before the law of sines was proposed by Snell and Descartes.

Al-Haitham’s book Kitab al-Manazir was translated into Latin in the Middle Ages, as was also his book dealing with the colors of sunset.  He dealt at length with the theory of various physical phenomena such as the rainbow, shadows, eclipses, and speculated on the physical nature of light.  Virtually all of the medieval Western writers on optics based their optical work on al-Haitham’s Opticae Thesaurus.  His work also influenced Leonardo da Vinci and Johannes Kepler.  His approach to optics generated fresh ideas and resulted in great progress in experimental methods.

Al-Haitham was the first to describe accurately the various parts of the eye and gave a scientific explanation of the process of vision.  He contradicted Ptolemy’s and Euclid’s theory of vision that the eye sends out visual rays to the object of the vision.  According to al-Haitham, the rays originate in the object of vision and not the eye. 

Al-Haitham also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon.  He is known for the earliest use of the camera obscura.  Through these extensive researches on optics, al-Haitham came to be considered the Father of Modern Optics.

In al-Haitham’s writings, one finds a clear explanation of the development of scientific methods as developed and applied by the Muslims, the systematic observation of physical phenomena and their relationship to a scientific theory.  This was a major breakthrough in scientific methodology, as distinct from guess work, and placed scientific study on a sound foundation comprising systematic relationship between observation, hypothesis and verification.

His research in catoptrics focused on spherical and parabolic mirrors and spherical aberration.  He made the important observation that the ratio between the angle of incidence and refraction does not remain constant and investigated the magnifying power of a lens.  His catoptrics contains the important problem known as Alhazen’s problem.  It comprises drawing lines from two points in the plane of a circle meeting at a point on the circumference and making equal angles with the normal at that point.  This leads to an equation of the fourth degree.   Al-Hazen also solved the shape of an aplantic surface of reflection.

In his book Mizan al-Hikmah, al-Haitham discussed the density of the atmosphere and developed a relation between it and the height.  He also studied atmospheric refraction.  Al-Haitham discovered that the twilight only ceases or begins when the sun is nineteen degrees below the horizon and attempted to measure the height of the atmosphere on that basis.  He deduced the height of homogeneous atmosphere to be fifty-five miles.

Al-Haitham’s contribution to mathematics and physics is extensive.  In mathematics, he developed analytical geometry by establishing linkage between algebra and geometry.  In physics, he studied the mechanics of motion of a body and was the first to propose that a body move perpetually unless an external force stops it or changes its direction of motion.  This is strikingly similar to the first law of motion.  He has also discussed the theories of attraction between masses, and it appears that he was aware of the magnitude of acceleration due to gravity.

Alhazen wrote more than two hundred books, very few of which have survived.  His monumental treatise on optics has survived through its Latin translation.  During the Middle Ages, his books on cosmology were translated into Latin, Hebrew and other European languages.  Also, he wrote a book on the subject of evolution. 

Alhazen's influence on physical sciences in general, and optics in particular, has been held in high esteem and his ideas heralded in a new era in both theoretical and experimental optical research.  He wrote commentaries on Aristotle, Galen, Euclid and Ptolemy.  Beer and Medler, in their famous work Der Mond, named one of the surface features of the Moon after Alhazen.  It is the name of a ring shaped plain to the West of the hypothetical Mare Crisium.  Additionally, on February 7, 1999, an asteroid was discovered by S. Sposetti at Gnosca, Italy.  The asteroid was named 59239 Alhazen.

Alhazen, the great Muslim scientist, died in 1039 in Cairo, Egypt. 

Abu ‘Ali al-Hasan ibn al-Haytham see Ibn al-Haytham
Haithem, al- see Ibn al-Haytham
Alhazen see Ibn al-Haytham
Avennathan see Ibn al-Haytham
The First Scientist see Ibn al-Haytham
Father of Modern Optics see Ibn al-Haytham