What Did Newton Contribute to Optics?
Newton's prism experiments, the theory of white light, the reflecting telescope, the Opticks, and the dispute with Hooke over the wave/particle interpretation of light.
What Did Newton Contribute to Optics?
Of Isaac Newton’s many contributions to natural philosophy, the one that first made him famous in England was his work on light and color. The prism experiments that he conducted in the 1660s, the reflecting telescope that he built in 1668, and the Opticks of 1704 were, in their own way, as important as the Principia. They established the experimental study of light as a serious branch of natural philosophy, and they led to one of the longest-running debates in this history: the debate over whether light is a particle or a wave. This page looks at Newton’s contributions to optics: the prism experiments and the theory of white light, the reflecting telescope, the Opticks of 1704, and the dispute with Hooke over the nature of light. The biographical context is on the Newton page. The discovery of universal gravitation is treated in How Did Newton Discover Gravity?, and the contents of the Principia in What Is in Newton’s Principia Mathematica?. The broader history of the telescope as an instrument is in the Telescope and Microscope.
The Aristotelian Theory of Color
To understand what Newton did, it helps to see the theory of light and color that he was attacking. The Aristotelian theory, which had dominated European natural philosophy since antiquity, held that light is a quality of the transparent medium — that is, of the air, water, glass, and other substances through which it passes — and that color is a modification of light by the medium. White was the natural color of light, when it passes through a transparent medium; colors arise when the light is mixed with darkness, or when the medium modifies the light in some way. Red, for instance, was supposed to be light mixed with a small amount of darkness; violet was light mixed with a large amount of darkness. The theory was qualitative, not quantitative, and it was, vague. The theory had been attacked, in the seventeenth century, by a number of writers. Descartes had argued that light is a pressure transmitted through a subtle fluid, the “ether,” and that color is a rotation of the particles of the fluid. Robert Hooke had argued, in his Micrographia of 1665, that light is a vibration of the ether, and that color is a property of the vibration. The theories were different, but both treated light as a phenomenon in the medium, not as a thing in itself. Newton’s contribution was to argue, on the basis of careful experiments, that white light is a mixture of the colors of the spectrum, and that each color is refracted by a glass prism at a slightly different angle. The argument was the first careful experimental analysis of light, and it was the foundation of the modern science of optics.
The Prism Experiments
Newton’s first experiments with prisms were conducted in 1665 and 1666, in his rooms at Trinity College, Cambridge. He was, in his own account, trying to repeat the famous experiment of the prismatic spectrum that had been performed by various natural philosophers in the preceding decades. The standard account said that when a beam of sunlight is passed through a glass prism, it emerges as a colored band — red at one end, violet at the other, with the other colors in between. Newton was puzzled by the fact that the colored band was much longer than it should have been, given the geometry of the prism. The standard Aristotelian account attributed the elongation of the spectrum to the modification of the light by the glass, in some way that depended on the angle of incidence. Newton, by a series of careful experiments, showed that this was not the case. He showed that the elongation of the spectrum was not due to the angle of incidence on the first face of the prism, or to any irregularity in the glass, but to a property of the light itself. Each color of the spectrum, he argued, had a different “refrangibility” — a different angle of refraction when it passed through the glass. Red was the least refrangible, violet the most, and the other colors were in between. The experimentum crucis, the “crucial experiment,” was the following. Newton took a beam of sunlight, passed it through a prism, and allowed the spectrum to fall on a board. He then cut a small hole in the board, allowing only a single color of the spectrum — say, the red — to pass through. He then passed this monochromatic red light through a second prism. The second prism refracted the red light, but it did not change its color. The same was true for any other color of the spectrum. And when he recombined the colors of the spectrum — by passing them through a lens, or by allowing them to fall on a white surface at the right angle — he got back white light. The conclusion, Newton argued, was that white light is a mixture of the colors of the spectrum, and that each color is refracted by the prism at a different angle. The argument was devastating to the Aristotelian theory. The Aristotelian theory held that color is a modification of light by the medium, and that the modification produces a continuous range of colors. The prism experiments showed that the colors of the spectrum are simple, that they are not produced by the modification of white light, and that they can be recombined to produce white light. The theory that white light is a mixture, not a simple, was a new claim, and it was the foundation of the modern theory of color.
The Theory of White Light
The theoretical claim that Newton made was that white light is a mixture of the colors of the spectrum, and that each color is a simple, homogeneous kind of light. The claim had two important consequences. First, it implied that the colors of objects are produced by selective reflection or transmission: a red apple is red because it reflects the red component of the white light that falls on it, and absorbs the other components. A blue piece of cloth is blue because it reflects the blue component. And so on. The theory explained, in a unified way, the colors of objects and the colors of the spectrum. Second, it implied that there is a kind of correspondence between the color of light and its refrangibility. The red end of the spectrum, which is the least refrangible, has a different refrangibility from the violet end, which is the most refrangible. The refrangibility is a property of the light, and it is the same property that determines the color. The theory was, in Newton’s hands, a unified theory of color and refraction. The theory of white light was, more important than the dispute over the wave/particle interpretation. The theory of white light was, in essence, the modern theory, and it has been a part of the standard account of color since the eighteenth century. The wave/particle dispute, by contrast, was not resolved until the twentieth century, with the growth of quantum mechanics, and the resolution was not along the lines that either Newton or Hooke had imagined.
The Reflecting Telescope
Newton’s work on optics led, in 1668, to the construction of a reflecting telescope. The standard refracting telescope of the period, in which the image is formed by a lens, suffered from a serious problem: the lens dispersed the different colors of light at different angles, producing a colored halo around the image, called chromatic aberration. The chromatic aberration was a serious limitation on the performance of refracting telescopes, and it was the limiting factor on the power of the telescope. Newton’s idea was to use a mirror, instead of a lens, to form the image. A mirror reflects all colors of light at the same angle, so it does not suffer from chromatic aberration. The reflecting telescope that Newton built was small — about six inches long, with a mirror about one inch in diameter — but it performed well. Newton showed it to the Royal Society in 1671, and the Society elected him a fellow in 1672. The design of the reflecting telescope became, over the next two centuries, the dominant design for large astronomical telescopes, and the giant reflecting telescopes of the modern era are direct descendants of Newton’s 1668 design. The construction of the reflecting telescope was a striking example of the way theory and practice could be combined in the new science. The theory of white light suggested that the chromatic aberration of the refracting telescope was a fundamental limitation, and the reflecting telescope was the practical solution. The combination of theory and practice was the new model of natural philosophy, and Newton’s telescope was one of its early successes.
The Opticks (1704)
The full expression of Newton’s work on optics was the Opticks, published in 1704, in English, and in 1706, in a Latin translation. The book was a complement to the Principia: it set out Newton’s experimental work on light and color, and it included, in the famous “Queries” at the end, a series of speculations about the nature of light, the structure of matter, and the possibility of a unified physics. The structure of the Opticks is unusual. The book is divided into three books, the first two of which are a detailed exposition of Newton’s experiments on the refraction, reflection, and diffraction of light. The third book is a set of “Observations” on the phenomena, and the famous “Queries” — a series of questions, more than thirty in all, that Newton used to explore the implications of his work. The Queries are the most interesting part of the book. They are speculative, ranging over a wide variety of topics, and they reveal Newton’s thinking on a number of issues that he did not develop in print. Query 1 asks whether the rays of light are corpuscles emitted by luminous bodies, or vibrations of the ether. Query 8 asks whether light is a particle or a wave, and considers the possibility that it is both. Query 23 asks whether the small particles of bodies have certain powers, forces, or virtues, by which they act at a distance. Query 31 asks whether there is a universal ether that transmits light, heat, and gravitation. The Queries were, in their own time, controversial, and they were the most influential part of the book. They were widely read and quoted, and they inspired a long line of speculation about the nature of light, the structure of matter, and the possibility of a unified physics. The Queries were the manifesto of the Newtonian natural philosophy, and they remained influential for two centuries.
The Dispute with Hooke
The most famous episode in Newton’s optical work was the dispute with Robert Hooke over the nature of light. The dispute began in 1672, when Newton sent the Royal Society his paper on the prism experiments. Hooke, in his capacity as Curator of Experiments, was asked to review the paper, and he wrote a long, critical response. Hooke argued that Newton’s experiments did not support his conclusions, and he proposed an alternative theory of light, in which light is a vibration of the ether, and color is a property of the vibration. The dispute went on for several years. Newton wrote a series of replies, and Hooke wrote a series of counter-replies. The dispute was a clash of temperaments: Newton was a careful experimentalist who insisted on the strict interpretation of the data, while Hooke was a brilliant speculator who was more interested in the theoretical implications. The dispute was also, a clash of theories: Newton proposed a corpuscular theory of light, in which light is a stream of particles, while Hooke proposed a wave theory, in which light is a vibration of the ether. The dispute was not resolved in the seventeenth century. The corpuscular theory of light was, dominant in the eighteenth century, partly because of Newton’s prestige. The wave theory was revived in the early nineteenth century, by Thomas Young and Augustin-Jean Fresnel, and it became the dominant theory in the nineteenth century. The two theories were eventually combined, in the twentieth century, in the wave-particle duality of quantum mechanics, which was a synthesis that neither Newton nor Hooke could have imagined. The dispute with Hooke was one of the formative episodes of Newton’s career. It taught him, he said, “to make no more loos attacks upon the publick.” It also, perhaps, contributed to Newton’s reluctance to publish his work on other topics, including the gravitational theory. See the biographies of Newton, and it is the subject of a substantial scholarly literature.
The Legacy of Newton’s Optics
Newton’s contributions to optics were, as important as his contributions to mechanics. The theory of white light was, in essence, the modern theory, and it has been a part of the standard account of color since the eighteenth century. The reflecting telescope became, in the eighteenth and nineteenth centuries, the dominant design for large astronomical telescopes. The Opticks was, for two centuries, the standard reference in optical theory. The Queries inspired a long line of speculation about the nature of light and the structure of matter. The full reception of Newton’s optics, and its incorporation into the broader Newtonian synthesis, is the central narrative of the Newton page and of the Telescope and Microscope. The way Newton’s optics combined with the Principia to form the Newtonian natural philosophy is one of the great stories in this history.