The Book of Optics 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 or Alhacen (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 proposed the modern intromission theory, the now accepted model that vision takes place by light entering the eye.

The book is also noted for its early use of the scientific method, its description of the camera obscura, and its formulation of Alhazen’s problem.

The book extensively affected the development of optics, physics, and mathematics in Europe between the 13th and 17th centuries.

Vision theory

Before the Book of Optics was written, two theories of vision existed. The extramission or emission theory was forwarded by the mathematicians Euclid and Ptolemy, who asserted that certain forms of radiation are emitted from the eyes onto the object which is being seen.

When these rays reached the object they allowed the viewer to perceive its color, shape and size. An early version of the intromission theory, held by the followers of Aristotle and Galen, argued that sight was caused by agents, which were transmitted to the eyes from either the object or from its surroundings.

Al-Haytham offered many reasons against the extramission theory, pointing to the fact that eyes can be damaged by looking directly at bright lights, such as the sun.

He claimed the low probability that the eye can fill the entirety of space as soon as the eyelids are opened as an observer looks up into the night sky.

Using the intromission theory as a foundation, he formed his own theory that an object emits rays of light from every point on its surface which then travel in all directions, thereby allowing some light into a viewer’s eyes.

According to this theory, the object being viewed is considered to be a compilation of an infinite number of points, from which rays of light are projected.

Light and color theory

In the Book of Optics, al-Haytham claimed the existence of primary and secondary light, with primary light being the stronger or more intense of the two.

The book describes how the essential form of light comes from self-luminous bodies and that accidental light comes from objects that obtain and emit light from those self-luminous bodies.

According to Ibn al-Haytham, 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 the primary light. Both primary and secondary light travels in straight lines.

Transparency is a characteristic of a body that can transmit light through them, such as air and water, although nobody 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 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, traveling in straight lines in both cases.

Al-Haytham presented many experiments in Optics that upheld his claims about light and its transmission. He also claimed that color acts much like light, being a distinct quality of a form and traveling from every point on an object in straight lines. Through experimentation, he concluded that color cannot exist without air.

Anatomy of the eye and visual process

As objects radiate light in straight lines in all directions, the eye must also be hit with this light over its outer surface. This idea presented a problem for al-Haytham and his predecessors as if this was the case, the rays received by the eye from every point on the object would cause a blurred image.

Al-Haytham solved this problem using his theory of refraction. He argued that although the object sends an infinite number of rays of light to the eye, only one of these lines falls on the eye perpendicularly: the other rays meet the eye at angles that are not perpendicular.

According to al-Haytham, this causes them to be refracted and weakened. He claimed that all the rays other than the one that hits the eye perpendicularly are not involved in vision.

In al-Haytham’s structure of the eye, the crystalline humor is the part that receives light 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.

Influence

The Book of Optics was most strongly influenced by Ptolemy‘s Optics, while the description of the anatomy and physiology of the eye was based upon an account by Galen.

The Book of Optics was translated into Latin by an unknown scholar at the end of the 12th (or the beginning of the 13th) century. The work was influential during the Middle Ages.

It was printed by Friedrich Risner in 1572, as part of his collection Opticae thesaurus. This included a book on twilight falsely attributed to Alhazen, as well as a work on optics by Witelo.

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*This article was originally published at en.wikipedia.org.