# History of optics

## The beginnings of geometrical optics

The Greek term τα όπτικά referred specifically to matters of vision[1], and hence early optics was concerned with the problem of how we see. The early writers discussed here treated vision more as a geometrical than as a physical, physiological, or psychological problem.

The first known author of a treatise on optics was the geometer Euclid (c. 325 BC–265 BC). Euclid began his study of optics as he began his study of geometry, with a set of self-evident axioms.

1. Lines (or visual rays) can be drawn in a straight line to the object.
2. Those lines falling upon an object form a cone.
3. Those things upon which the lines fall are seen.
4. Those things seen under a larger angle appear larger.
5. Those things seen by a higher ray, appear higher.
6. Right and left rays appear right and left.
7. Things seen within several angles appear clearer.

Euclid did not define the physical nature of these visual rays but, using the principles of geometry, he discussed the effects of perspective and the rounding of things seen at a distance.

Where Euclid had limited his analysis to simple direct vision, Hero of Alexandria (c. AD 10-70 ) extended the principles of geometrical optics to consider problems of reflection (catoptrics). Unlike Euclid, Hero occasionally commented on the physical nature of visual rays, indicating that they proceeded at great speed from the eye to the object seen and were reflected from smooth surfaces but could become trapped in the porosities of unpolished surfaces.[2] This has come to be known as emission theory.

Hero demonstrated the equality of the angle of incidence and reflection on the grounds that this is the shortest path from the object to the observer. On this basis, he was able to define the fixed relation between an object and its image in a plane mirror. Specifically, the image appears to be as far behind the mirror as the object really is in front of the mirror.

Like Hero, Ptolemy (c. 90 – c. 168) considered the visual rays as proceeding from the eye to the object seen, but, unlike Hero, considered that the visual rays were not discrete lines, but formed a continuous cone. Ptolemy extended the study of vision beyond direct and reflected vision; he also studied vision by refracted rays (dioptrics), when we see objects through the interface between two media of different density. He conducted experiments to measure the path of vision when we look from air to water, from air to glass, and from water to glass and tabulated the relationship between the incident and refracted rays.[3]

His tabulated results have been studied for the air water interface, and in general the values he obtained reflect the theoretical refraction given by modern theory, but the outliers are clearly distorted to represent Ptolemy's a priori model of the nature of refraction.

## Optics in the Islamic world

Al-Kindi (c. 801–873) was one of the earliest important optical writers in the Islamic world. In a work known in the west as De radiis stellarum, al-Kindi developed a theory "that everything in the world ... emits rays in every direction, which fill the whole world."[4] It is believed that this theory of the active power of rays influenced such Western scholars as Robert Grosseteste and Roger Bacon.[citation needed]

Ibn Sahl (c. 940-1000) was a mathematician associated with the court of Baghdad. About 984 he wrote a treatise On Burning Mirrors and Lenses in which he set out his understanding of how curved mirrors and lenses bend and focus light. In his work he discovered a law of refraction mathematically equivalent to Snell's law.[5] He used his law of refraction to compute the shapes of lenses and mirrors that focus light at a single point on the axis.

Ibn al-Haytham (known in Western Europe as Alhacen or Alhazen) (965-1040), often regarded as the "father of optics",[6] formulated "the first comprehensive and systematic alternative to Greek optical theories."[7] His key achievement was twofold: first, to insist that vision only occurred because of rays entering the eye and that rays postulated to proceed from the eye had nothing to do with it; the second was to define the physical nature of the rays discussed by earlier geometrical optical writers, considering them as the forms of light and color. He developed a camera obscura to demonstrate that light and color from different candles passed through a single aperture in straight lines, without intermingling at the aperture.[8] He then analyzed these physical rays according to the principles of geometrical optics. Ibn al-Haytham also employed the experimental scientific method as a form of demonstration in optics. He wrote many books on optics, most significantly the Book of Optics (Kitab al Manazir in Arabic), translated into Latin as the De aspectibus or Perspectiva, which disseminated his ideas to Western Europe and had great influence on the later developments of optics.[9] Another aspect associated with Ibn al-Haytham's optical research is related to systemic and methodological reliance on experimentation (i'tibar) and controlled testing in his scientific inquiries. Moreover, his experimental directives rested on combining classical physics ('ilm tabi'i) with mathematics (ta'alim; geometry in particular)in terms of devising the rudiments of what may be designated as a hypothetico deductive procedure in scientific research. This mathematical-physical approach to experimental science supported most of his propositions in Kitab al-Manazir (The Optics; De aspectibus or Perspectivae) and gounded his theories of vision, light and colour, as well as his research in catoptrics and dioptrics. His legacy was further advanced through the 'reforming' of his Optics by Kamal al-Din al-Farisi (d. ca. 1320) in the latter's Kitab Tanqih al-Manazir (The Revision of [Ibn al-Haytham's] Optics).[10][11]

## The western Middle Ages

The English bishop, Robert Grosseteste (c. 1175 - 1253), wrote on a wide range of scientific topics at the time of the origin of the medieval university and the recovery of the works of Aristotle. Grosseteste reflected a period of transition between the Platonism of early medieval learning and the new Aristotelianism, hence he tended to apply mathematics and the Platonic metaphor of light in many of his writings. He has been credited with discussing light from four different perspectives: an epistemology of light, a metaphysics or cosmogony of light, an etiology or physics of light, and a theology of light.[12]

Setting aside the issues of epistemology and theology, Grosseteste's cosmogony of light describes the origin of the universe in what may loosely be described as a medieval "big bang" theory. Both his biblical commentary, the Hexaemeron (1230 x 35), and his scientific On Light (1235 x 40), took their inspiration from Genesis 1:3, "God said, let there be light", and described the subsequent process of creation as a natural physical process arising from the generative power of an expanding (and contracting) sphere of light.[13]

File:Grosseteste-optics.jpg
Optical diagram showing light being refracted by a spherical glass container full of water. (from Roger Bacon or Robert Grosseteste)

His more general consideration of light as a primary agent of physical causation appears in his On Lines, Angles, and Figures where he asserts that "a natural agent propagates its power from itself to the recipient" and in On the Nature of Places where he notes that "every natural action is varied in strength and weakness through variation of lines, angles and figures."[14]

The English Franciscan, Roger Bacon (c. 1214 – 1294) was strongly influenced by Grosseteste's writings on the importance of light. In his optical writings (the Perspectiva, the De multiplicatione specierum, and the De speculis comburentibus) he cited a wide range of recently translated optical and philosophical works, including those of Alhacen, Aristotle, Avicenna, Averroes, Euclid, al-Kindi, Ptolemy, Tideus, and Constantine the African. Although he was not a slavish imitator, he drew his mathematical analysis of light and vision from the writings of the Arabic writer, Alhacen. But he added to this the Neoplatonic concept, perhaps drawn from Grosseteste, that every object radiates a power (species) by which it acts upon nearby objects suited to receive those species.[15] Note that Bacon's optical use of the term "species" differs significantly from the genus / species categories found in Aristotelian philosophy.

Another English Franciscan, John Pecham (died 1292) built on the work of Bacon, Grosseteste, and a diverse range of earlier writers to produce what became the most widely used textbook on Optics of the Middle Ages, the Perspectiva communis. His book centered on the question of vision, on how we see, rather than on the nature of light and color. Pecham followed the model set forth by Alhacen, but interpreted Alhacen's ideas in the manner of Roger Bacon.[16]

Like his predecessors, Witelo (c. 1230 - 1280 x 1314) drew on the extensive body of optical works recently translated from Greek and Arabic to produce a massive presentation of the subject entitled the Perspectiva. His theory of vision follows Alhacen and he does not consider Bacon's concept of species, although passages in his work demonstrate that he was influenced by Bacon's ideas. Judging from the number of surviving manuscripts, his work was not as influential as those of Pecham and Bacon, yet his importance, and that of Pecham, grew with the invention of printing.[17]

## Renaissance and early modern optics

Johannes Kepler(1571 – 1630)

René Descartes (1596 – 1650)

Christiaan Huygens (1629 – 1695)

Isaac Newton (1643 – 1727)

## Lenses and Lensmaking

The earliest known lenses were made from polished crystal, often quartz, and have been dated as early as 700 BC for Assyrian lenses such as the Layard / Nimrud lens.[18] There are many similar lenses from ancient Egypt, Greece and Babylon. The ancient Romans and Greeks filled glass spheres with water to make lenses. Glass lenses were not thought of until the 13th century. This is when Roger Bacon used parts of glass spheres as magnifying glasses and recommended them to be used to help people read. Roger Bacon got his inspiration from Alhacen in the 10th century. He discovered that light reflects from objects and does not get released from them. Between the 11th and 13th century "reading stones" were invented. Often used by monks to assist in illuminating manuscripts, these were primitive plano-convex lenses initially made by cutting a glass sphere in half. As the stones were experimented with, it was slowly understood that shallower lenses magnified more effectively.

## Notes

1. Oxford English Dictionary, [1]
2. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 14-15.
3. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), p. 16; A. M. Smith, Ptolemy's search for a law of refraction: a case-study in the classical methodology of 'saving the appearances' and its limitations, Arch. Hist. Exact Sci. 26 (1982), 221-240; Ptolemy's procedure is reported in the fifth chapter of his Optics.
4. Cited in D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), p. 19.
5. R. Rashed, "A Pioneer in Anaclastics: Ibn Sahl on Burning Mirrors and Lenses", Isis 81 (1990): 464–91.
6. R. L. Verma "Al-Hazen: father of modern optics", Al-Arabi, 8 (1969): 12-13.
7. D. C. Lindberg, "Alhazen's Theory of Vision and its Reception in the West", Isis, 58 (1967), p. 322.
8. David C. Lindberg, "The Theory of Pinhole Images from Antiquity to the Thirteenth Century," Archive for History of the Exact Sciences, 5(1968):154-176.
9. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 58-86.
10. Nader El-Bizri, "A Philosophical Perspective on Alhazen’s Optics," Arabic Sciences and Philosophy, Vol. 15, Issue 2 (2005), pp. 189-218 (Cambridge University Press)
11. Nader El-Bizri, "Ibn al-Haytham," in Medieval Science, Technology, and Medicine: An Encyclopedia, eds. Thomas F. Glick, Steven J. Livesey, and Faith Wallis (New York — London: Routledge, 2005), pp. 237-240.
12. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 94-99.
13. R. W. Southern, Robert Grosseteste: The Growth of an English Mind in Medieval Europe, (Oxford: Clarendon Press, 1986), pp. 136-9, 205-6.
14. A. C. Crombie, Robert Grosseteste and the Origins of Experimental Science, (Oxford: Clarendon Press, 1971), p. 110
15. D. C. Lindberg, "Roger Bacon on Light, Vision, and the Universal Emanation of Force," pp. 243-275 in Jeremiah Hackett, ed., Roger Bacon and the Sciences: Commemorative Essays, (Leiden: Brill, 1997), pp. 245-250; Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 107-18; The Beginnings of Western Science, (Chicago: Univ. of Chicago Pr., 1992, p. 313.
16. D. C. Lindberg, John Pecham and the Science of Optics: Perspectiva communis, (Madison, Univ. of Wisconsin Pr., 1970), pp. 12-32; Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 116-18.
17. D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago: Univ. of Chicago Pr., 1976), pp. 118-20.

## References

• Crombie, A. C. Robert Grosseteste and the Origins of Experimental Science. Oxford: Clarendon Press, 1971.
• Lindberg, D. C. "Alhazen's Theory of Vision and its Reception in the West", Isis 58 (1967), 321-341.
• Lindberg, D. C. Theories of Vision from al-Kindi to Kepler. Chicago: University of Chicago Press, 1976.
• Temple, R. The Crystal Sun. London: Arrow Books, 2000 ISBN 0-09-925679-7.
• History of Optics (audio mp3) by Simon Schaffer, Professor in History and Philosophy of Science at the University of Cambridge, Jim Bennett, Director of the Museum of the History of Science at the University of Oxford and Emily Winterburn, Curator of Astronomy at the National Maritime Museum (recorded by the BBC).