Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham (965 in Basra – c. 1040 in Cairo) was a Muslim[5] scientist, polymath, mathematician, astronomer and philosopher, described in various sources as either an Arab or Persian.[1][6] He made significant contributions to the principles of optics, as well as to astronomy, mathematics, visual perception, and to the scientific method. He also wrote insightful commentaries on works by Aristotle, Ptolemy, and the Greek mathematician Euclid.[7]He is frequently referred to as Ibn al-Haytham, and sometimes as al-Basri (Arabic: البصري), after his birthplace in the city of Basra.[8] He was also nicknamed Ptolemaeus Secundus ("Ptolemy the Second")[9] or simply "The Physicist"[10] in medieval Europe.
Born circa 965, in Basra, present-day Iraq, he lived mainly in Cairo, Egypt, dying there at age 74.[9] According to one version of his biography, overconfident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile.[11] After being ordered by Al-Hakim bi-Amr Allah, the sixth ruler of the Fatimid caliphate, to carry out this operation, he quickly perceived the impossibility of what he was attempting to do. Fearing for his life, he feigned madness[1][12] and was placed under house arrest, during which he undertook scientific work. After the death of Al-Hakim he was able to prove that he was not mad, and for the rest of his life he made money copying texts while writing mathematical works and teaching.[13] He is known as the "Father of Modern Optics, Experimental physics and Scientific methodology"[14][15][16][17] and could be regarded as the first theoretical physicist
Biography
Alhazen was born in Basra, in the Iraq province of the Buyid Empire.[1] He probably died in Cairo, Egypt. During the Islamic Golden Age, Basra was a "key beginning of learning",[18] and he was educated there and in Baghdad, the capital of the Abbasid Caliphate, and the focus of the "high point of Islamic civilization".[18] During his time in Buyid Iran, he worked as what could be described as a civil servant and studied maths and science.[8][19]One account of his career has him called to Egypt by Al-Hakim bi-Amr Allah, ruler of the Fatimid Caliphate, to regulate the flooding of the Nile, a task requiring an early attempt at building a dam at the present site of the Aswan Dam.[20] After deciding the scheme was impractical and fearing the caliph's anger, he feigned madness.
He was kept under house arrest from 1011 until al-Hakim's death in 1021.[21] During this time, he wrote his influential Book of Optics. After his house arrest ended, he wrote scores of other treatises on physics, astronomy and mathematics. He later traveled to Islamic Spain. During this period, he had ample time for his scientific pursuits, which included optics, mathematics, physics, medicine, and practical experiments. Some biographers have claimed that Alhazen fled to Syria, ventured into Baghdad later in his life, or was in Basra when he pretended to be insane. In any case, he was in Egypt by 1038.[8] During his time in Cairo, he contributed to the work of Dar-el-Hikma, the city's "House of Wisdom".[22]Among his students were Sorkhab (Sohrab), a Persian student who was one of the greatest people of Iran's Semnan and was his student for over 3 years, and Abu al-Wafa Mubashir ibn Fatek, an Egyptian scientist who learned mathematics from Alhazan.[23]
Legacy
Alhazen made significant improvements in optics, physical science, and the scientific method. Alhazen's work on optics is credited with contributing a new emphasis on experiment. The Latin translation of his main work, Kitab al-Manazir (Book of Optics),[24] exerted a great influence on Western science: for example, on the work of Roger Bacon, who cites him by name.[25] His research in catoptrics (the study of optical systems using mirrors) centred on spherical and parabolic mirrors and spherical aberration. He made the observation that the ratio between the angle of incidence and refraction does not remain constant, and investigated the magnifying power of a lens. His work on catoptrics also contains the problem known as "Alhazen's problem".[26]
Meanwhile in the Islamic world, Alhazen's work influenced Averroes' writings on optics,[27] and his legacy was further advanced through the 'reforming' of his Optics by Persian scientist Kamal al-Din al-Farisi (d. ca. 1320) in the latter's Kitab Tanqih al-Manazir (The Revision of [Ibn al-Haytham's] Optics).[28] He wrote as many as 200 books, although only 55 have survived, and many of those have not yet been translated from Arabic.[citation needed] Some of his treatises on optics survived only through Latin translation. During the Middle Ages his books on cosmology were translated into Latin, Hebrew and other languages. The crater Alhazen on the Moon is named in his honour,[29] as was the asteroid 59239 Alhazen.[30] In honour of Alhazen, the Aga Khan University (Pakistan) named its Ophthalmology endowed chair as "The Ibn-e-Haitham Associate Professor and Chief of Ophthalmology".[31] Alhazen (by the name Ibn al-Haytham) is featured on the obverse of the Iraqi 10,000 dinars banknote issued in 2003,[32] and on 10 dinar notes from 1982. A research facility that UN weapons inspectors suspected of conducting chemical and biological weapons research in Saddam Hussein's Iraq was also named after him.[32][33]
Book of Optics
Alhazen's most famous work is his seven volume treatise on optics, Kitab al-Manazir (Book of Optics), written from 1011 to 1021. Optics was translated into Latin by an unknown scholar at the end of the 12th century or the beginning of the 13th century.[34] It was printed by Friedrich Risner in 1572, with the title Opticae thesaurus: Alhazeni Arabis libri septem, nuncprimum editi; Eiusdem liber De Crepusculis et nubium ascensionibus (English : Optics treasure: Arab Alhazeni seven books, published for the first time: The book of the Twilight of the clouds and ascensions).[35] Risner is also the author of the name variant "Alhazen"; before Risner he was known in the west as Alhacen, which is the correct transcription of the Arabic name.[36] This work enjoyed a great reputation during the Middle Ages. Works by Alhazen on geometric subjects were discovered in the Bibliothèque nationale in Paris in 1834 by E. A. Sedillot. Other manuscripts are preserved in the Bodleian Library at Oxford and in the library of Leiden.
Theory of Vision
Two major theories on vision prevailed in classical antiquity. The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light. The second theory, the intromission theory supported by Aristotle and his followers, had physical forms entering the eye from an object. Previous Islamic writers (such as al-Kindi) had argued essentially on Euclidean, Galenist, or Aristotelian lines; Alhazen's achievement was to come up with a theory which successfully combined parts of the mathematical ray arguments of Euclid, the medical tradition of Galen, and the intromission theories of Aristotle. Alhazen's intromission theory followed al-Kindi (and broke with Aristotle) in asserting that "from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point".[37]
This however left him with the problem of explaining how a coherent image was formed from many independent sources of radiation; in particular, every point of an object would send rays to every point on the eye. What Alhazen needed was for each point on an object to correspond to one point only on the eye.[37] He attempted to resolve this by asserting that only perpendicular rays from the object would be perceived by the eye; for any one point on the eye, only the ray which reached it directly, without being refracted by any other part of the eye, would be perceived. He argued using a physical analogy that perpendicular rays were stronger than oblique rays; in the same way that a ball thrown directly at a board might break the board, whereas a ball thrown obliquely at the board would glance off, perpendicular rays were stronger than refracted rays, and it was only perpendicular rays which were perceived by the eye. As there was only one perpendicular ray that would enter the eye at any one point, and all these rays would converge on the centre of the eye in a cone, this allowed him to resolve the problem of each point on an object sending many rays to the eye; if only the perpendicular ray mattered, then he had a one-to-one correspondence and the confusion could be resolved.[38] He later asserted (in book seven of the Optics) that other rays would be refracted through the eye and perceived as if perpendicular.[39]
His arguments regarding perpendicular rays do not clearly explain why only perpendicular rays were perceived; why would the weaker oblique rays not be perceived more weakly?[40] His later argument that refracted rays would be perceived as if perpendicular does not seem persuasive.[41] However, despite its weaknesses, no other theory of the time was so comprehensive, and it was enormously influential, particularly in Western Europe: "Directly or indirectly, his De Aspectibus inspired much of the activity in optics which occurred between the 13th and 17th centuries." [42] Kepler's later theory of the retinal image (which resolved the problem of the correspondence of points on an object and points in the eye) built directly on the conceptual framework of Alhazen.[42]Alhazen showed through experiment that light travels in straight lines, and carried out various experiments with lenses, mirrors, refraction, and reflection.[26] He was the first to consider separately the vertical and horizontal components of reflected and refracted light rays, which was an important step in understanding optics geometrically.[43]
The camera obscura was known to the Chinese, and Aristotle had discussed the principle behind it in his Problems, however it is Alhazen's work which contains the first clear description[44] and early analysis[45] of the device. Alhazen studied the process of sight, the structure of the eye, image formation in the eye, and the visual system. Ian P. Howard argued in a 1996 Perception article that Alhazen should be credited with many discoveries and theories which were previously attributed to Western Europeans writing centuries later. For example, he described what became in the 19th century Hering's law of equal innervation; he had a description of vertical horopters which predates Aguilonius by 600 years and is actually closer to the modern definition than Aguilonius's; and his work on binocular disparity was repeated by Panum in 1858.[46]
Craig Aaen-Stockdale, while agreeing that Alhazen should be credited with many advances, has expressed some caution, especially when considering Alhazen in isolation from Ptolemy, who Alhazen was extremely familiar with. Alhazen corrected a significant error of Ptolemy regarding binocular vision, but otherwise his account is very similar; Ptolemy also attempted to explain what is now called Hering's law.[47] In general, Alhazen built on and expanded the optics of Ptolemy.[48][49] In a more detailed account of Ibn al-Haytham's contribution to the study of binocular vision based on Lejeune[50] and Sabra,[11] Raynaud[51] showed that the concepts of correspondence, homonymous and crossed diplopia were in place in Ibn al-Haytham's optics. But contrary to Howard, he explained why Ibn al-Haytham did not give the circular figure of the horopter and why, by reasoning experimentally, he was in fact closer to the discovery of Panum's fusional area than that of the Vieth-Müller circle. In this regard, Ibn al-Haytham's theory of binocular vision faced two main limits: the lack of recognition of the role of the retina, and obviously the lack of an experimental investigation of ocular tracts.
Alhazen's most original contribution was that after describing how he thought the eye was anatomically constructed, he went on to consider how this anatomy would behave functionally as an optical system.[52] His understanding of pinhole projection from his experiments appears to have influenced his consideration of image inversion in the eye,[53] which he sought to avoid.[54] He maintained that the rays that fell perpendicularly on the lens (or glacial humor as he called it) were further refracted outward as they left the glacial humor and the resulting image thus passed upright into the optic nerve at the back of the eye.[55] He followed Galen in believing that the lens was the receptive organ of sight, although some of his work hints that he thought the retina was also involved.[56].[15]
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