The beauty and majesty of rainbows have inspired awe in humans for millennia, but it wasn’t until Isaac Newton’s groundbreaking work revealing the secrets of light that we truly began to understand how they come together. form.
In this excerpt from the new book “Beautiful Experiments: An Illustrated History of Experimental Science” (The University of Chicago Press, 2023), science writer Philip Ball explains how Isaac Newton’s ingenious experiment with prisms transformed our understanding light.
The enigma of the rainbow was solved in the 17th century thanks to the work of the scientist considered by some to be the greatest who ever lived. In 1666, Isaac Newton, then a 23-year-old Cambridge graduate, carried out an experiment with light that transformed our understanding of it.
While the bar of rainbow colors – called a spectrum – produced when white light (like sunlight) passes through a glass prism was thought to be caused by a property of the prism that changes the light, Newton showed that colors are already inherent in the prism. the light itself. Legend has it that Newton had the experiment at his family home in Woolsthorpe, Lincolnshire, where he had returned to escape the great plague that ravaged England in 1665.
After all, it didn’t require any fancy apparatus – just a few prisms, which could be bought almost like trinkets in markets (although he needed good quality ones!). Although there is some truth to this, Newton had been planning such experiments for some time in his room at Cambridge: the plague should not be given credit for spurring this progress in the understanding of optics. Newton did not report his results until six years later, when he sent a review to the Royal Society of London, the intellectual center of “experimental philosophy” at mid-century.
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He was notoriously reluctant to divulge the results of his studies and had to be persuaded to write his famous laws of motion and his theories on the motions of the planets in his masterpiece, the Principia Mathematica, in 1687. The book in which he recorded his experiments and theories of light, Opticks, were finally published in 1704. This was not so much because Newton was suspicious of his work; on the contrary, he was rather stingy and very sensitive to criticism.
Newton begins his 1672 account by recounting his surprise to find that the colored spectrum produced by his prism was rectangular rather than circular in shape, “as the received laws of refraction” would lead one to expect. This seems a rather trivial question, especially to lead to such profound conclusions. In fact, his “surprise” is difficult to credit, because this prism effect was well known, notably to Newton himself, who had been fascinated by such instruments since his childhood.
Newton was undoubtedly engaging here in what is now standard practice in scientific articles: constructing a retrospective history in such a way as to give an understandable narrative arc to a description of experiences that might have a more random and perhaps even more random genesis. a totally different goal. Regardless, Newton embarked on an extensive program of experimentation to understand the effect of the prism on light.
We can imagine him almost literally playing with prisms, screens and lenses until he finds a configuration that allows him to formulate and study precise hypotheses. (Newton once said that “I feign no hypotheses,” but in truth, one can hardly do science without them.)
This is a common situation in experimental science: you may want to study a phenomenon without knowing exactly what the right questions are, much less how to deploy your instruments and measuring devices to answer them. You must develop a perception of the system you are trying to study.
Newton closed the “shutters” of his room, allowing a single, narrow ray of sunlight to pass through a hole in the prism. In this crucial experiment, Newton studied the nature of light after it leaves the prism. If the light becomes colored because of a transformation produced by the prism, then passing through a second prism can be expected to change the light again.
Newton used a board with a hole in it to filter out the entire spectrum except for a single color – red, for example – and then allowed that colored light to pass through the second prism. He discovered that this light emerged from the second prism refracted – bent at an angle – but otherwise unchanged. In other words, a prism only appears to bend (refract) light, leaving it otherwise unchanged. But this happens to different degrees (i.e. at different angles) for different colors.
This in itself was not new: the Anglo-Irish scientist Robert Boyle had said so in his 1664 book “Experiments and Considerations Touching Colors”, which Newton had read. But only Newton understood what this implied: refraction is then all there is to do.
The colors themselves are already in white light and the prism only separates them. As he put it, “light is made up of differently refrangible” (i.e. refractible) rays. The colors of the spectrum “are therefore not qualifications (alterations) of light… (as is generally believed), but original and connate properties”. It was a bold interpretation: the light of the sun was not, so to speak, elementary, but composed.
To test this idea, Newton used a lens to refocus a multi-colored spectrum into a single merged beam – which, he observed, was white. He also passed this reconstituted beam through another prism to reveal that it could once again be split into a spectrum as before.
Newton explained how his observations could explain the rainbow, produced by the refraction and reflection of light through raindrops that act like tiny prisms. The colors of everyday objects, he added, appear because they reflect “one kind of light more than another.”
And the results explained the defects in lenses (Newton himself had become adept at making them by grinding glass), with the refraction of different colors producing a defocusing effect called chromatic aberration. Royal Society secretary Henry Oldenburg told Newton that his report had been greeted with “rare applause” when it was read at a meeting in February 1672. But not everyone liked it.
After the paper was published in Philosophical Transactions, the in-house curator of the experiments, Robert Hooke, who considered himself an expert in optics, presented several criticisms (which we can now see were wrong). Newton responded with great condescension, sparking a long-running feud between the two men.
One problem is that Newton’s experiments, despite their apparent simplicity, are not easy to reproduce: some, in England and abroad, have tried and failed. But they have stood the test of time, a testament to the power of experience to literally illuminate the unknown which, according to the philosopher of science Robert Crease, gives what is called Newton’s experimentum crucis “a kind of moral beauty.”
Reprinted with permission from Beautiful Experiments: An Illustrated History of Experimental Science by Philip Ball, published by The University of Chicago Press. © 2023 by Quarto Publishing plc. All rights reserved.