Editing Ibn al-Haytham (section)

== ''Book of Optics'' ==
{{Main|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.<ref>{{Harvnb|Al-Khalili|2015}}.</ref> In it, Ibn al-Haytham was the first to explain that vision occurs when light reflects from an object and then passes to one's eyes,<ref name="Adamson 2016 77"/> and to argue that vision occurs in the brain, pointing to observations that it is subjective and affected by personal experience.{{sfn|Baker|2012|p=445}}

''Optics'' was [[Latin translations of the 12th century|translated into Latin]] by an unknown scholar at the end of the 12th century or the beginning of the 13th century.<ref>{{harvnb|Crombie|1971|p=147, n. 2}}.</ref>{{ efn| A. Mark Smith has determined that there were at least two translators, based on their facility with Arabic; the first, more experienced scholar began the translation at the beginning of Book One, and handed it off in the middle of Chapter Three of Book Three. {{harvnb|Smith|2001}} '''91''' Volume 1: Commentary and Latin text pp.xx–xxi. See also his 2006, 2008, 2010 translations.}}

This work enjoyed a great reputation during the [[Middle Ages]]. The Latin version of ''De aspectibus'' was translated at the end of the 14th century into Italian vernacular, under the title ''De li aspecti''.<ref>{{Cite journal |author=[[Enrico Narducci]] | title=Nota intorno ad una traduzione italiana fatta nel secolo decimoquarto del trattato d'ottica d'Alhazen |journal=Bollettino di Bibliografia e di Storia delle Scienze Matematiche e Fisiche | year=1871 | volume=4 |pages=1–40}}. On this version, see {{harvnb|Raynaud|2020|pp=139–153}}.</ref>

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: Treasury of Optics: seven books by the Arab Alhazen, first edition; by the same, on twilight and the height of clouds).<ref>{{Citation|url=http://www.mala.bc.ca/~mcneil/cit/citlcalhazen1.htm |title=Alhazen (965–1040): Library of Congress Citations | publisher=Malaspina Great Books |access-date=23 January 2008 |url-status=dead |archive-url=https://web.archive.org/web/20070927190009/http://www.mala.bc.ca/~mcneil/cit/citlcalhazen1.htm |archive-date=27 September 2007 }}{{verify source|date=February 2016}}</ref>
Risner is also the author of the name variant "Alhazen"; before Risner he was known in the west as Alhacen.<ref>{{harvnb|Smith|2001|p=xxi}}.</ref>
Works by Alhazen on geometric subjects were discovered in the [[Bibliothèque nationale]] in [[Paris]] in 1834 by E. A. Sedillot. In all, A. Mark Smith has accounted for 18 full or near-complete manuscripts, and five fragments, which are preserved in 14 locations, including one in the [[Bodleian Library]] at [[Oxford]], and one in the library of [[Bruges]].<ref>{{harvnb|Smith|2001|p=xxii}}.</ref>

=== Theory of optics ===
{{See also|Horopter}}
[[File:Thesaurus opticus Titelblatt.jpg|thumb|upright|Front page of the ''Opticae Thesaurus'', which included the first printed Latin translation of Alhazen's ''Book of Optics''. The illustration incorporates many examples of optical phenomena including perspective effects, the rainbow, mirrors, and refraction.]]
Two major theories on vision prevailed in [[classical antiquity]]. The first theory, the [[Emission theory (vision)|emission theory]], was supported by such thinkers as [[Euclid]] and [[Ptolemy]], who believed that sight worked by the [[eye]] emitting [[Ray (optics)|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. The strongest influence on the ''Book of Optics'' was from Ptolemy's [[Ptolemy#Optics|''Optics'']], while the description of the anatomy and physiology of the eye was based on Galen's account.<ref>{{harvnb|Smith|2001|p=lxxix}}.</ref> Alhazen's achievement was to come up with a theory that 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".<ref name="{{harvnb|lindberg|1976|p=73}}.">{{harvnb|Lindberg|1976|p=73}}.</ref> This 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.<ref name="{{harvnb|lindberg|1976|p=73}}." /> He attempted to resolve this by asserting that the eye would only perceive perpendicular rays from the object{{snd}}for any one point on the eye, only the ray that 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.<ref>{{harvnb|Lindberg|1976|p=74}}</ref> He later asserted (in book seven of the ''Optics'') that other rays would be refracted through the eye and perceived ''as if'' perpendicular.<ref>{{harvnb|Lindberg|1976|p=76}}</ref> 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?<ref>{{harvnb|Lindberg|1976|p=75}}</ref> His later argument that refracted rays would be perceived as if perpendicular does not seem persuasive.<ref>{{harvnb|Lindberg|1976|pages=76–78}}</ref> 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'' ([[Book of Optics]]) inspired much activity in optics between the 13th and 17th centuries. [[Kepler]]'s later theory of the [[retina]]l 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.<ref>{{harvnb|Lindberg|1976|p=86}}.</ref>

Alhazen showed through experiment that light travels in straight lines, and carried out various experiments with [[lens (optics)|lenses]], [[mirror]]s, [[refraction]], and [[Reflection (physics)|reflection]].<ref name="auto">{{harvnb|Al Deek|2004}}.</ref> His analyses of reflection and refraction considered the vertical and horizontal components of light rays separately.<ref>{{harvnb|Heeffer|2003}}.</ref>

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 (journal)|Perception]]'' article that Alhazen should be credited with many discoveries and theories 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 wrote a description of vertical [[horopter]]s 600 years before [[Aguilonius]] that is actually closer to the modern definition than Aguilonius's{{snd}}and his work on [[binocular disparity]] was repeated by Panum in 1858.<ref>{{harvnb|Howard|1996}}.</ref> 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]], with whom Alhazen was extremely familiar. 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.<ref>{{harvnb|Aaen-Stockdale|2008}}</ref> In general, Alhazen built on and expanded the optics of Ptolemy.<ref>{{harvnb|Wade|1998|pages=240, 316, 334, 367}}; {{harvnb|Howard|Wade|1996|pages=1195, 1197, 1200}}.</ref>

In a more detailed account of Ibn al-Haytham's contribution to the study of binocular vision based on Lejeune<ref>{{harvnb|Lejeune|1958}}.</ref> and Sabra,<ref name="{{harvnb|sabra|1989}}.">{{harvnb|Sabra|1989}}.</ref> Raynaud<ref>{{harvnb|Raynaud|2003}}.</ref> 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.

[[File:Alhazen1652.png|left|thumb|upright|The structure of the [[human eye]] according to Ibn al-Haytham. Note the depiction of the [[optic chiasm]]. —Manuscript copy of his [[Kitāb al-Manāẓir]] (MS Fatih 3212, vol. 1, fol. 81b, [[Süleymaniye Mosque]] Library, Istanbul)]]
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.<ref>{{harvnb|Russell|1996|p=691}}.</ref> His understanding of [[Pinhole camera model|pinhole projection]] from his experiments appears to have influenced his consideration of image inversion in the eye,<ref>{{harvnb|Russell|1996|p=689}}.</ref> which he sought to avoid.<ref>{{harvnb|Lindberg|1976|pages= 80–85}}</ref> 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.<ref>{{harvnb|Smith|2004|pages=186, 192}}.</ref> He followed [[Galen]] in believing that the [[Lens (anatomy)|lens]] was the receptive organ of sight, although some of his work hints that he thought the [[retina]] was also involved.<ref>{{harvnb|Wade|1998|p=14}}</ref>

Alhazen's synthesis of light and vision adhered to the Aristotelian scheme, exhaustively describing the process of vision in a logical, complete fashion.<ref>{{Cite journal|url=http://www.jstor.org/stable/3657357|title=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 "Kitāb al-Manāẓir": Volume Two|author=Smith, A. Mark|year=2001|journal=Transactions of the American Philosophical Society|volume=91|issue=5|pages=339–819|doi=10.2307/3657357|jstor=3657357|access-date=12 January 2015|archive-date=30 June 2015|archive-url=https://web.archive.org/web/20150630235046/http://www.jstor.org/stable/3657357?|url-status=live}}</ref>

His research in [[catoptrics]] (the study of optical systems using mirrors) was centred on spherical and [[Parabola|parabolic]] mirrors and [[spherical aberration]]. He made the observation that the ratio between the [[angle of incidence (optics)|angle of incidence]] and [[refraction]] does not remain constant, and investigated the [[Magnification|magnifying]] power of a [[Lens (optics)|lens]].<ref name="auto" />

=== Law of reflection ===
{{Main|Specular reflection}}
Alhazen was the first physicist to give complete statement of the law of reflection.<ref>{{Cite book |last=Stamnes |first=J. J. |url=https://books.google.com/books?id=dGQ-DwAAQBAJ&dq=alhazen+law+of+reflection&pg=PT15 |title=Waves in Focal Regions: Propagation, Diffraction and Focusing of Light, Sound and Water Waves |date=2017 |publisher=Routledge |isbn=978-1-351-40468-6 |language=en |access-date=22 February 2023 |archive-date=31 March 2023 |archive-url=https://web.archive.org/web/20230331171120/https://books.google.com/books?id=dGQ-DwAAQBAJ&dq=alhazen+law+of+reflection&pg=PT15 |url-status=live }}</ref><ref>{{Cite book |last=Mach |first=Ernst |url=https://books.google.com/books?id=7dPCAgAAQBAJ&dq=alhazen+incident+ray+reflected+ray+lie+on+same+plane&pg=PA29 |title=The Principles of Physical Optics: An Historical and Philosophical Treatment |date=2013 |publisher=Courier Corporation |isbn=978-0-486-17347-4 |language=en |access-date=22 February 2023 |archive-date=31 March 2023 |archive-url=https://web.archive.org/web/20230331172406/https://books.google.com/books?id=7dPCAgAAQBAJ&dq=alhazen+incident+ray+reflected+ray+lie+on+same+plane&pg=PA29 |url-status=live }}</ref><ref>{{Cite book |last=Iizuka |first=Keigo |url=https://books.google.com/books?id=h9n6CAAAQBAJ&dq=alhazen+law+of+reflection&pg=PA7 |title=Engineering Optics |date=2013 |publisher=Springer Science & Business Media |isbn=978-3-662-07032-1 |language=en |access-date=22 February 2023 |archive-date=31 March 2023 |archive-url=https://web.archive.org/web/20230331171118/https://books.google.com/books?id=h9n6CAAAQBAJ&dq=alhazen+law+of+reflection&pg=PA7 |url-status=live }}</ref> He was first to state that the incident ray, the reflected ray, and the normal to the surface all lie in a same plane perpendicular to reflecting plane.{{sfn|Selin|2008|p=1817}}<ref>{{Cite book |last=Mach |first=Ernst |url=https://books.google.com/books?id=7dPCAgAAQBAJ&dq=alhazen+first+incident+ray+reflected+ray+lie+on+same+plane&pg=PA29 |title=The Principles of Physical Optics: An Historical and Philosophical Treatment |date=2013 |publisher=Courier Corporation |isbn=978-0-486-17347-4 |language=en |access-date=22 February 2023 |archive-date=31 March 2023 |archive-url=https://web.archive.org/web/20230331171118/https://books.google.com/books?id=7dPCAgAAQBAJ&dq=alhazen+first+incident+ray+reflected+ray+lie+on+same+plane&pg=PA29 |url-status=live }}</ref>

=== Alhazen's problem ===
{{Main|Alhazen's problem}}
[[File:Theorem of al-Haitham.JPG|thumb|The [[Circle#Theorems|theorem of Ibn Haytham]]]]
His work on [[catoptrics]] in Book V of the Book of Optics contains a discussion of what is now known as Alhazen's problem, first formulated by [[Ptolemy]] in 150 AD. It comprises drawing lines from two points in the [[plane (mathematics)|plane]] of a circle meeting at a point on the [[circumference]] and making equal angles with the [[Normal (geometry)|normal]] at that point. This is equivalent to finding the point on the edge of a circular [[billiard table]] at which a player must aim a cue ball at a given point to make it bounce off the table edge and hit another ball at a second given point. Thus, its main application in optics is to solve the problem, "Given a light source and a spherical mirror, find the point on the mirror where the light will be reflected to the eye of an observer." This leads to an [[Quartic equation|equation of the fourth degree]].<ref>{{harvnb|O'Connor|Robertson|1999}}, {{harvnb|Weisstein|2008}}.</ref> This eventually led Alhazen to derive a formula for the sum of [[fourth power]]s, where previously only the formulas for the sums of squares and cubes had been stated. His method can be readily generalized to find the formula for the sum of any integral powers, although he did not himself do this (perhaps because he only needed the fourth power to calculate the volume of the paraboloid he was interested in). He used his result on sums of integral powers to perform what would now be called an [[Integral|integration]], where the formulas for the sums of integral squares and fourth powers allowed him to calculate the volume of a [[paraboloid]].<ref>{{harvnb|Katz|1995|pp=165–169, 173–174}}.</ref> Alhazen eventually solved the problem using [[conic section]]s and a geometric proof. His solution was extremely long and complicated and may not have been understood by mathematicians reading him in Latin translation.
Later mathematicians used [[Descartes]]' analytical methods to analyse the problem.<ref>{{harvnb|Smith|1992}}.</ref> An algebraic solution to the problem was finally found in 1965 by Jack M. Elkin, an actuarian.<ref>{{Citation|last=Elkin|first=Jack M.|title=A deceptively easy problem|journal=Mathematics Teacher|volume=58|issue=3|pages=194–199|year=1965|doi=10.5951/MT.58.3.0194|jstor=27968003}}</ref> Other solutions were discovered in 1989, by Harald Riede<ref>{{Citation|last=Riede|first=Harald|title=Reflexion am Kugelspiegel. Oder: das Problem des Alhazen|journal=Praxis der Mathematik|volume=31|issue=2|pages=65–70|year=1989|language=de}}</ref> and in 1997 by the [[University of Oxford|Oxford]] mathematician [[Peter M. Neumann]].<ref>{{Citation|last=Neumann|first=Peter M.|author-link=Peter M. Neumann|title=Reflections on Reflection in a Spherical Mirror|journal=[[American Mathematical Monthly]]|volume=105|issue=6|pages=523–528|year=1998|jstor=2589403|mr=1626185|doi=10.1080/00029890.1998.12004920}}</ref><ref>{{Citation|last=Highfield |first=Roger |author-link=Roger Highfield |date=1 April 1997 |title=Don solves the last puzzle left by ancient Greeks |journal=[[Electronic Telegraph]] |volume=676 |url=https://www.telegraph.co.uk/htmlContent.jhtml?html=/archive/1997/04/01/ngre01.html|url-status=dead |archive-url=https://web.archive.org/web/20041123051228/http://www.telegraph.co.uk/htmlContent.jhtml?html=%2Farchive%2F1997%2F04%2F01%2Fngre01.html |archive-date=23 November 2004 }}</ref>
Recently, [[Mitsubishi Electric Research Laboratories]] (MERL) researchers solved the extension of Alhazen's problem to general rotationally symmetric quadric mirrors including hyperbolic, parabolic and elliptical mirrors.<ref>{{harvnb|Agrawal|Taguchi|Ramalingam|2011}}.</ref>

=== Camera Obscura ===
The [[camera obscura]] was known to the [[History of Science and Technology in China|ancient Chinese]], and was described by the [[Han Chinese]] [[polymath]] [[Shen Kuo]] in his scientific book ''[[Dream Pool Essays]]'', published in the year 1088 C.E. [[Aristotle]] had discussed the basic principle behind it in his ''Problems'', but Alhazen's work contained the first clear description of [[camera obscura]].<ref>{{harvnb|Kelley|Milone|Aveni|2005|p=83}}: "The first clear description of the device appears in the ''Book of Optics'' of Alhazen."</ref> and early analysis<ref>{{harvnb|Wade|Finger|2001}}: "The principles of the camera obscura first began to be correctly analysed in the eleventh century, when they were outlined by Ibn al-Haytham."</ref> of the device.

Ibn al-Haytham used a [[camera obscura]] mainly to observe a partial solar eclipse.<ref>German physicist Eilhard Wiedemann first provided an abridged German translation of ''On the shape of the eclipse'': {{Cite journal |author=Eilhard Wiedemann |title=Über der Camera obscura bei Ibn al Haiṭam |journal=Sitzungsberichte phys.-med. Sozietät in Erlangen |year=1914 |volume=46 | pages=155–169}} The work is now available in full: {{harvnb|Raynaud|2016}}.</ref>
In his essay, Ibn al-Haytham writes that he observed the sickle-like shape of the sun at the time of an eclipse. The introduction reads as follows: "The image of the sun at the time of the eclipse, unless it is total, demonstrates that when its light passes through a narrow, round hole and is cast on a plane opposite to the hole it takes on the form of a moonsickle."

It is admitted that his findings solidified the importance in the history of the [[camera obscura]]<ref>{{Cite book|title=History of Photography|last=Eder|first=Josef|publisher=Columbia University Press|year=1945|location=New York|page=37}}</ref> but this treatise is important in many other respects.

Ancient optics and medieval optics were divided into optics and burning mirrors. Optics proper mainly focused on the study of vision, while burning mirrors focused on the properties of light and luminous rays. ''On the shape of the eclipse'' is probably one of the first attempts made by Ibn al-Haytham to articulate these two sciences.

Very often Ibn al-Haytham's discoveries benefited from the intersection of mathematical and experimental contributions. This is the case with ''On the shape of the eclipse''. Besides the fact that this treatise allowed more people to study partial eclipses of the sun, it especially allowed to better understand how the camera obscura works. This treatise is a physico-mathematical study of image formation inside the camera obscura. Ibn al-Haytham takes an experimental approach, and determines the result by varying the size and the shape of the aperture, the focal length of the camera, the shape and intensity of the light source.<ref>{{harvnb|Raynaud|2016|pp=130–160}}</ref>

In his work he explains the inversion of the image in the camera obscura,<ref>{{harvnb|Raynaud|2016|pp=114–116}}</ref> the fact that the image is similar to the source when the hole is small, but also the fact that the image can differ from the source when the hole is large. All these results are produced by using a point analysis of the image.<ref>{{harvnb|Raynaud|2016|pp=91–94}}</ref>

=== Refractometer ===
{{Main|Refractometer}}
In the seventh tract of his book of optics, Alhazen described an apparatus for experimenting with various cases of refraction, in order to investigate the relations between the angle of incidence, the angle of refraction and the angle of deflection. This apparatus was a modified version of an apparatus used by Ptolemy for similar purpose.<ref>{{Cite book |url=http://archive.org/details/history-of-science-and-technology-in-islam-fuat-sezgin |title=History Of Science And Technology In Islam Fuat Sezgin |date=2011}}</ref><ref>{{Cite book |last=Gaukroger |first=Stephen |url=https://books.google.com/books?id=QVwDs_Ikad0C&dq=ptolemy+alhazen+refractometer&pg=PA142 |title=Descartes: An Intellectual Biography |date=1995 |publisher=Clarendon Press |isbn=978-0-19-151954-3 |language=en}}</ref><ref>{{Cite book |last=Newton |first=Isaac |url=https://books.google.com/books?id=gNrLQN0VbAoC&dq=ptolemy+alhazen+refractometer&pg=PA175 |title=The Optical Papers of Isaac Newton|volume =1: The Optical Lectures 1670–1672 |date=1984|publisher=Cambridge University Press |isbn=978-0-521-25248-5 |language=en}}</ref>

=== Unconscious inference ===
{{Main|Unconscious inference}}
Alhazen basically states the concept of unconscious inference in his discussion of colour before adding that the inferential step between sensing colour and differentiating it is shorter than the time taken between sensing and any other visible characteristic (aside from light), and that "time is so short as not to be clearly apparent to the beholder." Naturally, this suggests that the colour and form are perceived elsewhere. Alhazen goes on to say that information must travel to the central nerve cavity for processing and:<blockquote>the sentient organ does not sense the forms that reach it from the visible objects until
after it has been affected by these forms; thus it does not sense color as color or light as light until after it has been affected by the form of color or light. Now the affectation received by the sentient organ from the form of color or of light is a certain change; and change must take place in time; .....and it is in the time during which the form extends from the sentient organ's surface to the cavity of the common nerve, and in (the time) following that, that the sensitive faculty, which exists in the whole of the sentient body will perceive color as color...Thus the last sentient's perception of color as such and of light as such takes place at a time following that in which the form arrives from the surface of the sentient organ to the cavity of the common nerve.<ref>{{Cite book |last1=Boudrioua |first1=Azzedine |url=https://books.google.com/books?id=WD0PEAAAQBAJ&dq=the+sentient+organ+does+not+sense+the+forms+that+reach+it+from+the+visible+objects+until+after+it+has+been+a&pg=PA76 |title=Light-Based Science: Technology and Sustainable Development, The Legacy of Ibn al-Haytham |last2=Rashed |first2=Roshdi |last3=Lakshminarayanan |first3=Vasudevan |date=2017 |publisher=CRC Press |isbn=978-1-4987-7940-1 |language=en}}</ref></blockquote>

=== Color constancy ===
{{Main|Color constancy}}
Alhazen explained [[color constancy]] by observing that the light reflected from an object is modified by the object's color. He explained that the quality of the light and the color of the object are mixed, and the visual system separates light and color. In Book II, Chapter 3 he writes:<blockquote>Again the light does not travel from the colored object to the eye unaccompanied by the color, nor does the form of the color pass from the colored object to the eye unaccompanied by the light. Neither the form of the light nor that of the color existing in the colored object can pass except as mingled together and the last sentient can only
perceive them as mingled together. Nevertheless, the sentient perceives that the visible object is luminous and that the light seen in the object is other than the color and that these are two properties.<ref>{{Cite book |last1=Boudrioua |first1=Azzedine |url=https://books.google.com/books?id=WD0PEAAAQBAJ&dq=Al-Haytham+described+color+constancy+by+observing+that+light+reflected+by+an+object+is+modified+by+the+color+of+the+object&pg=PA78 |title=Light-Based Science: Technology and Sustainable Development, The Legacy of Ibn al-Haytham |last2=Rashed |first2=Roshdi |last3=Lakshminarayanan |first3=Vasudevan |date=2017|publisher=CRC Press |isbn=978-1-4987-7940-1 |language=en}}</ref></blockquote>

=== Other contributions ===
The ''Kitab al-Manazir'' (Book of Optics) describes several experimental observations that Alhazen made and how he used his results to explain certain optical phenomena using mechanical analogies. He conducted experiments with [[projectile]]s and concluded that only the impact of [[perpendicular]] projectiles on surfaces was forceful enough to make them penetrate, whereas surfaces tended to deflect [[Oblique angle|oblique]] projectile strikes. For example, to explain refraction from a rare to a dense medium, he used the mechanical analogy of an iron ball thrown at a thin slate covering a wide hole in a metal sheet. A perpendicular throw breaks the slate and passes through, whereas an oblique one with equal force and from an equal distance does not.<ref>{{harvnb|Russell|1996|p=695}}.</ref> He also used this result to explain how intense, direct light hurts the eye, using a mechanical analogy: Alhazen associated 'strong' lights with perpendicular rays and 'weak' lights with oblique ones. The obvious answer to the problem of multiple rays and the eye was in the choice of the perpendicular ray, since only one such ray from each point on the surface of the object could penetrate the eye.<ref>{{harvnb|Russell|1996|p=}}.</ref>

Sudanese psychologist Omar Khaleefa has argued that Alhazen should be considered the founder of [[experimental psychology]], for his pioneering work on the psychology of visual perception and [[optical illusion]]s.<ref name="auto2">{{harvnb|Khaleefa|1999}}</ref> Khaleefa has also argued that Alhazen should also be considered the "founder of [[psychophysics]]", a sub-discipline and precursor to modern psychology.<ref name="auto2" /> Although Alhazen made many subjective reports regarding vision, there is no evidence that he used quantitative psychophysical techniques and the claim has been rebuffed.<ref>{{harvnb|Aaen-Stockdale|2008}}.</ref>

Alhazen offered an explanation of the [[Moon illusion]], an illusion that played an important role in the scientific tradition of medieval Europe.<ref>{{harvnb|Ross|Plug|2002}}.</ref> Many authors repeated explanations that attempted to solve the problem of the Moon appearing larger near the horizon than it does when higher up in the sky. Alhazen argued against Ptolemy's refraction theory, and defined the problem in terms of perceived, rather than real, enlargement. He said that judging the distance of an object depends on there being an uninterrupted sequence of intervening bodies between the object and the observer. When the Moon is high in the sky there are no intervening objects, so the Moon appears close. The perceived size of an object of constant angular size varies with its perceived distance. Therefore, the Moon appears closer and smaller high in the sky, and further and larger on the horizon. Through works by [[Roger Bacon]], [[John Pecham]] and Witelo based on Alhazen's explanation, the Moon illusion gradually came to be accepted as a psychological phenomenon, with the refraction theory being rejected in the 17th century.<ref>{{harvnb|Hershenson|1989|pp=9–10}}.</ref> Although Alhazen is often credited with the perceived distance explanation, he was not the first author to offer it. [[Cleomedes]] ({{circa}} 2nd century) gave this account (in addition to refraction), and he credited it to [[Posidonius]] ({{circa}} 135–50 BCE).<ref>{{harvnb|Ross|2000}}.</ref> Ptolemy may also have offered this explanation in his ''Optics'', but the text is obscure.<ref>{{harvnb|Ross|Ross|1976}}.</ref> Alhazen's writings were more widely available in the Middle Ages than those of these earlier authors, and that probably explains why Alhazen received the credit.