The beauty and majesty of rainbows have inspired awe in humans for millennia, but it wasn't until Isaac Newton's groundbreaking work unlocking the secrets of light did we truly begin to understand how they form.
In this extract 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 of light.
The puzzle of the rainbow was resolved in the seventeenth century through the work of the scientist who some regard as the greatest ever to have lived. In 1666, Isaac Newton — then a 23-year-old Cambridge graduate — performed an experiment with light that transformed our understanding of it.
While it was thought that the bar of rainbow colors — called a spectrum — produced when white light (like sunlight) travels through a glass prism is caused by some property of the prism that alters the light, Newton showed the colors are already inherent in the light itself. Legend has it that Newton did the experiment at his family home in Woolsthorpe, Lincolnshire, to which he had returned to escape the Great Plague that ravaged England in 1665.
It did not, after all, require any fancy apparatus — just a few prisms, which could be bought almost as trinkets at markets (although he needed good-quality ones!). While there's truth in that, Newton had been planning such experiments for a while in his Cambridge room: we need not credit the plague for stimulating this leap in understanding optics. Newton didn't report his results until six years later, when he sent an account to the Royal Society in London, the intellectual center of "experimental philosophy" in the mid-century.
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He was famously reluctant to disclose the outcomes of his studies, and had to be cajoled into writing down his celebrated laws of motion and theories about the motions of the planets in his masterwork the Principia Mathematica in 1687. The book in which he recorded his experiments and theories about light, Opticks, was finally published in 1704. This was not so much because Newton was diffident about his work; on the contrary, he was rather covetous about it, and highly sensitive to criticism.
Newton begins his 1672 account by relating his surprise that the colored spectrum produced by his prism was rectangular in shape rather than circular, "as the received laws of Refraction" would lead one to expect. It seems a rather trifling question, especially to lead to such profound conclusions. In fact, his "surprise" is hard to credit, for this effect of a prism was well known, not least to Newton himself, who had been fascinated with such instruments since he was a boy.
Newton was here no doubt indulging what is now a common practice in scientific papers: to construct a retrospective story so as to give a comprehensible narrative arc to a description of experiments that might have a more haphazard genesis and perhaps initially a different goal entirely. At any rate, Newton embarked on a thorough program of experimentation to figure out what the prism was doing to light.
One can imagine him almost literally playing with prisms, screens, and lenses until he found a configuration that allowed him to formulate and investigate some definite hypotheses. (Newton once famously claimed that "I feign no hypotheses," but in truth one can hardly do science at all without them.)
But only Newton saw what this implies: that refraction is then all there is to it
It's a common situation for experimental science: you might want to investigate a phenomenon but be unsure quite what the right questions are, let alone how to deploy your instruments and measuring devices to answer them. You need to develop a feeling for the system you're trying to study.
Newton closed the "window-shuts" of his room, admitting a single narrow beam of sunlight through a hole, which passed into the prism. In the crucial experiment, Newton investigated the nature of the light after it exited the prism. If the light became colored because of some transformation produced by the prism, then a passage through a second prism might be expected to alter the light again.
Newton used a board with a hole in it to screen off all the spectrum except for a single color — red, say — and then allowed that colored light to pass through the second prism. He found that this light emerged from the second prism refracted — bent at an angle — but otherwise unchanged. In other words, a prism seems only to bend (refract) light, leaving it otherwise unaltered. But it does so to different degrees (that is, at different angles) for different colors.
This in itself was nothing new: the Anglo-Irish scientist Robert Boyle had said as much in his 1664 book "Experiments and Considerations Touching Colours," which Newton had read. But only Newton saw what this implies: that refraction is then all there is to it.
The colors themselves are already in the white light, and all the prism does is to separate them out. As he put it, "Light consists of Rays differently refrangible" [meaning refractable]. The colors of the spectrum, then, "are not Qualifications [alterations] of Light … (as 'tis generally believed), but Original and connate properties." That was a bold interpretation: sunlight was not, so to speak, elemental, but compound.
To test this idea, Newton used a lens to refocus a many-hued 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 again be split into a spectrum just as before.
Newton explained how his observations could account for the rainbow, produced by the refraction and reflection of light through raindrops that act as tiny prisms. The colors of everyday objects, he added, arise because they reflect "one sort of light in greater plenty than another."
And the results explained the defects of lenses (Newton himself had become adept at making these by grinding glass), whereby refraction of different colors produces a defocusing effect called chromatic aberration. The Royal Society's secretary Henry Oldenburg told Newton that his report was met with "uncommon applause" when read at a gathering in February 1672. But not everyone appreciated it.
After the paper was published in the society's Philosophical Transactions, its in-house curator of experiments, Robert Hooke, who considered himself an expert on optics, presented several criticisms (which we can now see were mistaken). Newton replied with lofty condescension, igniting a long-standing feud between the two men.
One problem is that Newton's experiments, despite their apparent simplicity, are not easy to replicate: some, in England and abroad, tried and failed. But they have stood the test of time, a testament to the power of experiment to literally illuminate the unknown that, in the judgment of philosopher of science Robert Crease, gives Newton's so-called 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.
