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November 2014 Newton: ‘Opticks’

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Isaac Newton Opticks: Or, a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light  London, 1704  First edition

Reflection 1 Perhaps the key point of Newton’s observations was that white light is made up ow is a rainbow formed? Why of a superposition of many different “rays” do I see colors like magenta, of light with different colors. Furthermore, H yellow, and turquoise blue on an each individual color of light, in a oily puddle or when my kids blow soap continuous spectrum, has its own property bubbles? Why does a prism project the of “refrangibility,” or refraction. When a ray colors of the rainbow on the wall? And why of red light (such as a laser pointer) hits a are all the really big telescopes based on glass surface at an angle, it will be bent as it mirrors and not lenses, which was the passes into the new medium. A green laser technology used by Galileo? beam is bent slightly more, a blue one more Newton answered all of these questions still, and so on. This explains the working of and many more while performing a prism, spreading out the different colors experiments in optics during the latter part from the initial white light, and also is of the seventeenth century. Galileo had crucial in understanding the rainbow: As made great use of the refracting telescope light rays from the sun enter a raindrop, they (one with lenses) a century earlier, but he are refracted, reflected off the back of the did not have basic knowledge about the droplet, and bent again as they exit toward physical principles behind his instrument. our eyes. Kepler made some progress in that regard, Newton quickly realized that his but it was up to Newton, working with the experiments with prisms had an important previously elaborated law of refraction technological consequence. A lens can be (bending) of light rays, who synthesized thought of crudely as two prisms put several ideas into a comprehensive set of together base-to-base; therefore, blue and optical principles. red light will always focus differently, and any image made with a refracting telescope

1 must necessarily exhibit distortions in color light to explain the so-called photoelectric that effectively make the objects being effect, the work for which he was awarded observed seem blurred. His solution came the Nobel Prize. quickly in the form of a reflecting (now Like Kepler with his astrology activities, known as Newtonian) telescope based on the Newton is a bridge figure in the history of reflection of light from a mirror to an science. He is perhaps the physicist whose eyepiece for the observer. Since reflection of name is invoked more than any other in the light from a surface does not depend on the teaching of physics to students to this day, wavelength (i.e., color) of the incident light, mainly for his contributions to mechanics the aberrations due to refraction were and dynamics as put forth in the Principia eliminated. Mathematica. Aside from the important Newton believed that light was made up of contributions he made to the understanding tiny particles; therefore, his language of rays of the laws of motion and optical principles, was a reasonable one for describing the Newton also invented what we now call motion of those particles. During his calculus as a mathematical tool to aid in his investigation of refraction, he even posed other work. As if this were not enough, he the question as to whether the separation of spent a great deal of his time in the red and blue light might be due to a kind of laboratory performing alchemical curveball effect induced on the light experiments trying to transmute metals, for particles by the glass through which they example. But the subject to which he were passing, an idea that seems to have devoted the greatest part of his writings was come from watching the behavior of balls that of biblical exegesis, trying to construct being hit in a game of tennis. literal interpretations of biblical events. Newton made many observations of the properties of colored rings and bands of —Robert Brecha, PhD, Professor, Physics light formed by thin pieces of glass or thin films of oil, but he was not able to give a Reflection 2 theoretical explanation of these phenomena because of his particle view of light. A ptics was a well-established complete explanation and more definitive discipline by the seventeenth experiments would have to wait for the century. However, though the nineteenth century and the work of Thomas O production of mirrors, telescopes, Young and the electromagnetic wave theory microscopes, spectacles, and other optical of light developed by James Clerk Maxwell. instruments was well advanced, the methods Interestingly enough, however, at the were largely empirical, and there was no beginning of the twentieth century, Albert detailed understanding of the nature and Einstein returned to the particle theory of behavior of light. The construction of a

2 telescope, for example, was largely a matter about pulses of light (white as a uniform of trying different lenses until a suitable pulse and color as a combination of red and combination could be found. Ideas of color blue in a compounded or perturbed pulse). were particularly confused. The subject was From then onward, Hooke and Newton were ready for a more disciplined approach. This implacable enemies. was the essence of Isaac Newton’s Newton’s ideas on optics and the nature of contributions and the thrust of his Opticks. light and color were well advanced by this He had already written his great work time, but it was not until 1704—the year Philosophiae Naturalis Principia following Hooke’s death—that Opticks was Mathematica (Mathematical Principles of finally published: “To avoid being engaged Natural Philosophy) published in Latin in in Disputes about these Matters, I have 1687, which enunciated the principles of hitherto delayed the printing, and should still classical mechanics, removing all the have delayed it, had not the Importunity of irrational and ill-formed ideas that had Friends prevailed upon me.” slowed science’s advance. The same rational Newton’s method was to base everything approach appears in theOpticks— on actual measurements. Speculation in the significantly, in English (a Latin translation absence of evidence was futile. He appeared only later). established that there was a particular Science is a community pursuit with quality of light that was associated with its communication as its lifeblood. It thrives by color response and with the degree to which the sharing of information and ideas, and to it was refracted at a given surface, or its encourage this, the Royal Society was refrangibility. This quality was unalterable. established in 1662, while Newton was a The light itself was not colored. Color was a student at Trinity College, Cambridge. By human response to the stimulation by the 1667, he had made significant contributions light. He made it clear that if he spoke of in what we now call calculus, and he had light as possessing a color, he would be begun his work on gravitation. He had also speaking “accordingly to such Conceptions made great progress in his studies of color as vulgar People … would be apt to frame.” and applied his ideas to the improvement of He recognized the seven colors that we still telescopes by the use of reflecting rather associate with the spectrum; red, orange, than transmitting elements to reduce yellow, green, blue, indigo, and violet, with chromatic aberration. red being the least refrangible and the violet Newton reported his work on color and on the greatest. We have all seen the colors reflecting telescopes to the Royal Society in produced by sunlight in thin films of 1671, and, to his considerable dismay, he material, like oil floating on a puddle. was immediately ridiculed by Robert Hooke, Newton studied these colors in detail and who tried to advance his own vague ideas included in the Opticks techniques, still

3 completely valid, for accurate prediction of optical behavior, but also because it clearly the colors of thin films. demonstrated that it was possible to apply Nowadays, we try to make our logical reasoning to science and technology publications completely objective, and we to establish natural laws. appear to have lost the ability to include our The book reads as well today as it must thought processes. The Opticks vividly have in 1704. Not only is it one of the most recounts Newton’s attempts to understand significant texts in optics; it also paints a what his data were telling him. He struggled remarkable picture of the early development with the true nature of light itself and for of the scientific method. various reasons favored the corpuscular theory, although he tried hard to remain —H. Angus Macleod, DTech, neutral. University of Arizona Opticks was a very important publication not just for the unambiguous sets of rules of

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