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Some Readings in the Early History of Light

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Some Readings in the Early History of Light Paul Avery PHY 3400 Aug. 23, 1999 Some Readings in the Early History of Light The Greeks In optics, Euclid's textbook (called the Optics) set the precedent. Euclid postulated visual rays to be straight lines, and he defined the apparent size of an object in terms of the angle formed by the rays drawn from the top and the bottom of the object to the observer's eye. He then proved, for example, that nearer objects appear larger and appear to move faster and showed how to measure the height of distant objects from their shadows or reflected images, and so on. Other textbooks set out theorems on the phenomena of reflection and refraction (the field called catoptrics). The most extensive survey of optical phenomena is a treatise attributed to the astronomer Ptolemy (2nd century AD), which survives only in the form of an incomplete Latin translation (12th century) based on a lost Arabic translation. It covers the fields of geometric optics and catoptrics, as well as experimental areas, such as binocular vision, and more general philosophical principles (the nature of light, vision, and colour). Of a somewhat different sort are the studies of burning mirrors by Diocles (late 2nd century BC), who proved that the surface that reflects the rays from the Sun to a single point is a paraboloid of revolution. Constructions of such devices remained of interest as late as the 6th century AD, when Anthemius of Tralles, best known for his work as architect of the Hagia Sophia at Constantinople, compiled a survey of remarkable mirror configurations. (See "Optics".) "mathematics, history of" Encyclopædia Britannica Online. http://www.eb.com:180/bol/topic?eu=118175&sctn=15&pm=1 [Accessed 22 August 1999]. Alhazen of Arabia Arabic ABU 'ALI AL-HASAN IBN AL-HAYTHAM, mathematician and physicist who made the first significant contributions to optical theory since the time of Ptolemy (flourished 2nd century). In his treatise on optics, translated into Latin in 1270 as Opticae thesaurus Alhazeni libri vii, Alhazen published theories on refraction, reflection, binocular vision, focussing with lenses, the rainbow, parabolic and spherical mirrors, spherical aberration, atmospheric refraction, and the apparent increase in size of planetary bodies near the Earth's horizon. He was first to give an accurate account of vision, correctly stating that light comes from the object seen to the eye. "Alhazen" Encyclopædia Britannica Online. http://www.eb.com:180/bol/topic?eu=5788&sctn=1&pm=1 [Accessed 22 August 1999]. Medieval Studies of Light 1 The medieval world was caricatured by thinkers of the 18th-century Enlightenment as a period of darkness, superstition, and hostility to science and learning. On the contrary, it was one of great technological vitality. The advances that were made may appear today as trifling, but that is because they were so fundamental. They included the horseshoe and the horse collar, without which horsepower cannot be efficiently exploited. The invention of the crank, the brace and bit, the wheelbarrow, and the flying buttress made possible the great Gothic cathedrals. Improvements in the gear trains of waterwheels and the development of windmills harnessed these sources of power with great efficiency. Mechanical ingenuity, building on experience with mills and power wheels, culminated in the 14th century in the mechanical clock, which not only set a new standard of chronometrical accuracy but also provided philosophers with a new metaphor for nature itself. An equal amount of energy was devoted to achieving a scientific understanding of nature, but it is essential to understand to what use medieval thinkers put this kind of knowledge. As the fertility of the technology shows, medieval Europeans had no deep prejudices against utilitarian knowledge. But the areas in which scientific knowledge could find useful expression were few. Instead, science was viewed chiefly as a means of understanding God's creation and, thereby, the Godhead itself. The best example of this attitude is found in the medieval study of optics. Light, as Genesis makes clear, was among the first creations of God. The 12th-13th-century cleric-scholar Robert Grosseteste saw in light the first creative impulse. As light spread it created both space and matter, and, in its reflection from the outermost circle of the cosmos, it gradually solidified into the heavenly spheres. To understand the laws of the propagation of light was to understand, in some slight way, the nature of the creation. In the course of studying light, particular problems were isolated and attacked. What, for example, is the rainbow? It is impossible to get close enough to a rainbow to see clearly what is going on, for as the observer moves, so too does the rainbow. It does seem to depend upon the presence of raindrops, so medieval investigators sought to bring the rainbow down from the skies into their studies. Insight into the nature of the rainbow could be achieved by simulating the conditions under which rainbows occur. For raindrops the investigators substituted hollow glass balls filled with water, so that the rainbow could be studied at leisure. Valid conclusions about rainbows could then be drawn by assuming the validity of the analogy between raindrops and water-filled globes. This involved the implicit assumptions that nature was simple (i.e., governed by a few general laws) and that similar effects had similar causes. Such a nature was what could be expected of a rational, benevolent deity; hence, the assumption could be persuasively adopted. "science, history of" Encyclopædia Britannica Online. http://www.eb.com:180/bol/topic?eu=117480&sctn=11&pm=1 [Accessed 22 August 1999]. The 17th Century The science of optics in the 17th century expressed the fundamental outlook of the scientific revolution by combining an experimental approach with a quantitative analysis of phenomena. Optics had its origins in 2 Greece, especially in the works of Euclid (c. 300 BC), who stated many of the results in geometric optics that the Greeks had discovered, including the law of reflection: the angle of incidence is equal to the angle of reflection. In the 13th century, such men as Roger Bacon, Robert Grosseteste, and John Pecham, relying on the work of the Arab Alhazen (d. 1039), considered numerous optical problems, including the optics of the rainbow. It was Kepler, taking his lead from the writings of these 13th-century opticians, who set the tone for the science in the 17th century. Kepler introduced the point by point analysis of optical problems, tracing rays from each point on the object to a point on the image. Just as the mechanical philosophy was breaking the world into atomic parts, so Kepler approached optics by breaking organic reality into what he considered to be ultimately real units. He developed a geometric theory of lenses, providing the first mathematical account of Galileo's telescope. Descartes sought to incorporate the phenomena of light into mechanical philosophy by demonstrating that they can be explained entirely in terms of matter and motion. Using mechanical analogies, he was able to derive mathematically many of the known properties of light, including the law of reflection and the newly discovered law of refraction. "physical science" Encyclopædia Britannica Online. http://www.eb.com:180/bol/topic?eu=115385&sctn=9&pm=1 [Accessed 22 August 1999]. Sir Isaac Newton: The optics. Beginning with Kepler's Paralipomena in 1604, the study of optics had been a central activity of the scientific revolution. Descartes's statement of the sine law of refraction, relating the angles of incidence and emergence at interfaces of the media through which light passes, had added a new mathematical regularity to the science of light, supporting the conviction that the universe is constructed according to mathematical regularities. Descartes had also made light central to the mechanical philosophy of nature; the reality of light, he argued, consists of motion transmitted through a material medium. Newton fully accepted the mechanical nature of light, although he chose the atomistic alternative and held that light consists of material corpuscles in motion. The corpuscular conception of light was always a speculative theory on the periphery of his optics, however. The core of Newton's contribution had to do with colours. An ancient theory extending back at least to Aristotle held that a certain class of colour phenomena, such as the rainbow, arises from the modification of light, which appears white in its pristine form. Descartes had generalized this theory for all colours and translated it into mechanical imagery. Through a series of experiments performed in 1665 and 1666, in which the spectrum of a narrow beam was projected onto the wall of a darkened chamber, Newton denied the concept of modification and replaced it with that of analysis. Basically, he denied that light is simple and homogeneous--stating instead that it is complex and heterogeneous and that the phenomena of colours arise from the analysis of the heterogeneous mixture into its simple components. The ultimate source of Newton's conviction that light is corpuscular was his recognition that individual rays of 3 light have immutable properties; in his view, such properties imply immutable particles of matter. He held that individual rays (that is, particles of given size) excite sensations of individual colours when they strike the retina of the eye. He also concluded that rays refract at distinct angles--hence, the prismatic spectrum, a beam of heterogeneous rays, i.e., alike incident on one face of a prism, separated or analyzed by the refraction into its component parts--and that phenomena such as the rainbow are produced by refractive analysis. Because he believed that chromatic aberration could never be eliminated from lenses, Newton turned to reflecting telescopes; he constructed the first ever built.
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