Seventeenth-century Experimental and Theoretical Advances Regarding the Nature of Light Lay the Foundations of Modern Optics
Seventeenth-century Experimental and Theoretical Advances Regarding the Nature of Light Lay the Foundations of Modern Optics
Johannes Kepler's (1571-1630) early seventeenth-century researches on the nature of light were the culmination of medieval developments in the science of perspectiva and inaugurated a century of research that laid the foundations of modern optics. Willebrord Snell (1580-1626) shortly thereafter discovered the law of refraction, which allowed mathematical-physical theories of light to be developed in earnest, while René Descartes (1596-1650) developed a mechanistic wave-theory of light that did much to define the boundaries for future optical studies. Christiaan Huygens (1629-1695) was the first to successfully mathematize the wave picture, while Isaac Newton (1642-1727) developed a corpuscular theory. The two latter views were eventually synthesized in quantum theory during the early years of the twentieth century.
Theories about the nature and propagation of light in antiquity were intimately connected with theories of vision, and implicit in all theories of vision was the requirement that there be direct contact between the visual organ and objects of vision. Different accounts of how this contact occurred were promulgated and developed into opposing schools of thought in Ancient Greece.
The atomists adopted the position of Leucippus (c. 500-c. 450 b.c.), according to whom objects were thought to emit thin films or images of themselves through the intervening space to the eye. This "intromission" theory of vision was an alternative to the "extromission" theory championed by the Pythagoreans. Exponents of the extromission theory believed the eye emitted an invisible fire that "touched" objects of vision to reveal their colors and shape. Aristotle (384-322 b.c.) proposed a "mediumistic" theory whereby objects transmit their visible qualities through the intervening air to the eye.
Euclid (c. 330-c. 260 b.c.) developed the extromission theory in Optica (c. 300 b.c.), offering a geometrical theory of perspective in which the apparent size and shape of objects was determined by their distance and orientation with respect to an observer's line of sight. Ptolemy (c. a.d. 100-170) continued in this tradition, teaching the equality of the angles of incidence and reflection. He further maintained that the angles of the incident and refracted light rays had a constant relationship. Also during the second century, Galen (c. 130-c. 200) produced an alternate mediumistic theory. The purpose of Aristotle's mediumistic theory was to provide a physical explanation of how light was transmitted from visual objects to an observer, while Galen's was primarily designed to satisfy physiological criteria derived from the eye's anatomy.
When Greek optical works were translated into Arabic during the ninth century a.d., traditional distinctions were adopted and old arguments rehearsed. Only the work of Alhazen (c. 965-1038) decisively broke with the past. Alhazen proposed a new intromission theory that, for the first time, sought to simultaneously satisfy mathematical, physical, and physiological criteria. Exploiting the geometrical optics of Euclid and Ptolemy in conjunction with his knowledge of ocular anatomy, Alhazen explained the physical contact between an object and observer through intromitted rays. Though difficulties remained, the intromissionist character of vision was never again seriously challenged.
As Greek and Arabic texts became available in the Latin West during the twelfth and thirteenth centuries, the conflicting views of Aristotle and Alhazen held sway. Albertus Magnus (c. 1200-1280), the first great expositor of the Aristotelian corpus, defended the Aristotelian mediumistic theory, according to which light is a state of the medium that makes objects on the other side of it visible. Following Alhazen's lead, Roger Bacon (c. 1214-c. 1294) attempted to display the underlying unity of the major traditions by reconciling them. Most notably, he mathematized Robert Grosseteste's (c.1168-1253) Neoplatonic views on species transference through the medium and posited it as an explication of Aristotle's qualitative transformation of the medium. This produced the doctrine of multiplication of species.
Bacon argued that what was transferred was a series of simulacra called forth successively from the medium. He believed these likenesses were corporeal. Bacon's views circulated widely and helped establish the tradition of perspectiva in the West. They were developed in John Pecham's (c. 1230-1292) Perspectiva communis and Witelo's (c. 1230-c. 1277) Perspectiva. Nevertheless, Aristotelians were still in the majority by far.
New life was breathed into the perspectivist tradition during the Renaissance partly due to a growing interest in realistic painting. Kepler seized upon these ideas, summarizing and extending them in his own optical researches. In Ad Vitellionem paralipomena (1604) he developed a more satisfactory theory of vision—arguing the only way to establish a one-to-one correspondence between points in the visual field and points in the eye was if light rays were refracted through the eye's humors to focus on the retina as an inverted image. He also produced the first analysis of the telescope in Dioptrice (1611).
After Kepler it became generally accepted that light was not a modification of the transparent medium, rather that it existed as an independent thing whose properties could be inquired into. It was likewise accepted that light was emitted by luminous bodies and that it was rectilinearly propagated in rays. Furthermore, Kepler's methodological emphasis on the mathematical properties of reflection and refraction was widely adopted. In fact, it remains the basis of modern physical optics. Unfortunately, Kepler was unable to derive a mathematical law of refraction.
In 1621 Snell discovered the law of refraction. He demonstrated that the ratio of the sines of the angles of the incident and refracted rays to the normal remains constant. However, priority of publication goes to Descartes, who presented the law without proof in Dioptrique (1637) along with his wave theory of light.
Descartes's wave theory, despite its shortcomings, introduced a fruitful new area of study that many subsequent researchers took as the starting point for their own investigations. He asserted that light was a mechanical disturbance transmitted with infinite speed through the subtle matter filling the universe. However, in attempting to explain reflection and refraction, he posited a mechanical model that treated light as particles. When light particles strike a surface they are reflected elastically so that, in agreement with observation, the angle of incidence equals the angle of reflection. Descartes's corpuscular model also accounted for the quantitative law of refraction. However, it implied that light travels faster in denser media.
Pierre de Fermat (1601-1665) later showed that Snell's law of refraction could be deduced from the least-time principle, which implied that light travels slower in denser media. As striking as this result was, it was still generally believed that light was propagated instantaneously. In fact, Descartes stated that if light were not propagated instantaneously then he would be ready to confess that he knew "absolutely nothing." He avoided dealing with the problems raised by his corpuscular analysis by treating the model as a merely theoretical-educational device.
Newton took Descartes's model more seriously and developed a comprehensive corpuscular theory. By treating optical phenomena as a species of particle dynamics, Newton provided a plausible physical mechanism for light propagation. His was also the only seventeenth-century proposal to provide an adequate theory of colors. When his prism experiments of 1666 revealed white light was composed of different colors refracted through characteristic angles, Newton interpreted this to mean that white light was composed of streams of particles that were sorted and differently diverted to produce the spectrum of colors. After Newton published his theory in 1672, a multiyear controversy with Robert Hooke (1635-1703) ensued. In Micrographia (1665) Hooke had advocated a wavelike theory of light and spoke in general terms of the finite speed of light. However, he provided nothing comparable to Newton's theory of colors. It also seemed to Newton that wave theories were incapable of explaining the sharpness of shadows or the optical phenomena newly discovered by Erasmus Bartholin (1625-1698).
In 1669 Bartholin noticed that the mineral known as Iceland spar (calcite) produces double images of objects viewed through it. He assumed that light transmitted through the crystal was being refracted through different angles so as to produce two rays. He further noticed that the resulting beams could be split again but only for certain orientations. Huygens developed a wave theory of light that, in opposition to Newton, explained the initial double refraction. Though the latter effect—polarization—could not be explained by existing wave theories, it was eventually accounted for by nineteenth-century wave theories.
Huygens, influenced by Fermat's work, adopted the finite velocity of light as a hypothesis of his theory several years before Ole Römer provided a demonstration of this fact. In 1676 Römer noticed the intervals between successive eclipses of Jupiter's satellites varied depending on Earth's positions—diminishing as Earth approached and increasing as it receded. He correctly attributed this to the time required by light to travel the Jupiter-Earth distance. Based on his own estimate of Earth's orbital diameter, Huygens exploited Römer's finding to calculate light's velocity. Though his value of 140,000 miles (225,000 kilometers) per second is about 25% too small, it represented a considerable achievement.
Huygens presented his completed wave theory before the Académie des Sciences in 1679 but waited until 1690 to publish his Traité de la lumière. He conceived of light as a disturbance propagated by mechanical means at a finite speed through a subtle medium of closely packed elastic particles. According to Huygens's principle, a vibrating particle transfers its motion to those touching it in the direction of motion. Each particle so disturbed becomes the source of a hemispherical wave-front. Where many such fronts overlap, light is visible. Huygens's systematic treatment allowed his wave theory to be fruitfully applied and developed. With it he mathematically demonstrated the rectilinear propagation of light and deduced the laws of reflection and refraction.
Newton's corpuscular theory dominated eighteenth-century optical thinking. It was eclipsed by Huygens's wave theory in the early nineteenth century. The two views were later synthesized in the quantum theory of light during the early years of the twentieth century.
STEPHEN D. NORTON
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