Book of Optics
The Book of Optics (Arabic: Kitāb al-Manāẓir (كتاب المناظر); Latin: De Aspectibus or Perspectiva; Italian: Deli Aspecti) is a seven-volume treatise on optics and other fields of study composed by the medieval Arab scholar Ibn al-Haytham, known in the West as Alhazen (965– c. 1040 AD).
The Book of Optics presented experimentally founded arguments against the widely held extramission theory of vision (as held by Euclid in his Optica) and in favor of intromission theory, as supported by thinkers such as Aristotle, the now accepted model that vision takes place by light entering the eye.  Alhazen's work transformed the way in which light and vision was understood, earning him the title the "father of modern optics"
Before the writing of the Book of Optics there were two types of theories of vision that were held in contention. One was the extramission or emission theory. This theory was presented by the mathematicians Euclid and Ptolemy and asserted that certain forms of radiation are emitted from the eyes conically onto the object which is being seen. The striking of the rays on the object allow the viewer to perceive things such as the color, shape, and size of the object. The opposing theory was the intromission theory held by the followers of Aristotle and Galen which held that agents were transmitted to the eye from either the object or its surroundings and caused perception. Al-Haytham held the intromission theory of vision, offering many reasons against the extramission theory. He drew on the fact that eyes can be harmed and damaged by looking at very bright lights, such as the sun, directly and for a prolonged period of time, showing that light has an effect on the eye, not the other way around. He also claimed the high improbability of the ability of the eye to fill up the entire area of space and the stars the instant the eyelids are opened as an observer looks up into the sky. Using the intromission theory as a foundation, al-Haytham formed his own theory that an object being viewed emits rays of light from every point on the object which travel to the viewer's eye. According to his theory, the object being viewed is not considered as a whole object, but a compilation of an infinite amount of points that together compose the entirety of the object from which the rays of light are projected.
Light and color theory
In his Book of Optics, al-Haytham claims that there are two types of light, primary light and secondary light with primary light being the stronger or more intense of the two. He says that the essential form of light comes from self-luminous bodies and accidental light comes from objects that obtain and emit light from those self-luminous bodies. Primary light comes from self-luminous bodies and secondary light is the light that comes from accidental objects. Accidental light can only exist if there is a source of primary light. Both primary and secondary light travel in straight lines. He says transparency is a characteristic of those bodies that transmit light through them, such as air and water, although no body can completely transmit light or be entirely transparent. Opaque objects are those through which light cannot pass through directly, although there are degrees of opaqueness and transparency in an object which determine how much light can actually pass through. Opaque objects are struck with light and can become luminous bodies themselves which radiate secondary light. Light can be refracted by going through partially transparent objects and can also be reflected by striking smooth objects such as mirrors, travelling in straight lines in both cases. Al-Haytham presents many experiments in Optics that uphold his claims about light and its transmission. He also claims that color acts much like light, being a distinct quality of a form and travelling from every point on an object in straight lines. Through experimentation he concludes that color cannot exist without air.
Anatomy of the eye and visual process
Because objects radiate light in rectilinear motion in all directions, the eye must also be hit with this light at all points. The problem this presented to al-Haytham and his predecessors was that if this was the case, the result of all the lines of light hitting all the points on the eye from every point on the object would cause a very blurry and unorganized perception of the object. Al-Haytham presented a solution to this problem using his theory of refraction. He states that although the object sends an infinite amount of rays of light to the eye, only one of these lines falls on the eye perpendicularly. All the other rays come in contact with the eye at angles that aren't perpendicular causing them to be refracted and weakened. He claims that only the ray of light that hits the eye perpendicularly is strong enough to be seen, and all the other weaker rays play only a very minor part in vision, too small to be perceived. In his structure of the eye, the crystalline humor is the part of the eye that picks up the rays from the object and forms a visual cone with the object being perceived as the base of the cone and the center of the crystalline humor in the eye as the vertex. Other parts of the eye are the aqueous humor in front of the crystalline humor and the vitreous humor at the back. These, however, do not play as critical of a role in vision as the crystalline humor. The crystalline humor transmits the image it perceives to the brain through an optic nerve.
- Book I - Book I deals with al-Haytham's theories on light, colors, and vision.
- Book II - Book II is where al-Haytham presents his theory of visual perception.
- Book III & Book VI - Book III and Book VI present al-Haytham's ideas on the errors in visual perception with Book VI focusing on errors related to reflection.
- Book IV & Book V - Book IV and Book V provide experimental evidence for al-Haytham's theories on reflection.
- Book VII - Book VII deals with the concept of refraction.
The Book of Optics was translated into Latin by an unknown scholar at the end of the 12th century or the beginning of the 13th century. His work was very influential during the Middle Ages. It was printed by Friedrich Risner in 1572, in his collection Opticae thesaurus along with a book on twilight falsely attributed to Alhazen and Witelo's related book on optics.
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