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Where do rainbows come from?

Posted by Kevin Hartnett  July 22, 2013 09:14 AM

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Journalist and former Nature editor Philip Ball has a new book out called “Curiosity: How Science Became Interested in Everything.” It’s about the spirit of open-ended inquiry that animated the Scientific Revolution, and it includes a fun retelling of the quest to understand one of the most striking events in nature: the rainbow.

At the beginning of the 17th century, Ball explains, theories about rainbow formation were way off. The prevailing idea, which came from Aristotle, had it that rainbows occurred when light was reflected from a distant cloud, and that the different colors resulted from different ways in which light and dark were mixed.

During the 17th century, though, several of the biggest minds in the history of science moved rainbow theory incrementally in the right direction.

Johannes Kepler figured out that rainbows were not formed by light bouncing off of clouds, but rather light being reflected from inside raindrops. Descartes refined Kepler’s idea, explaining that rainbows were formed when light interacted with raindrops through a process of “two refractions and one reflection” (light refracts when it enters a raindrop, reflects off of the rear of the raindrop, and refracts again when it exits the raindrop). Descartes' realization explained why rainbows are arc-shaped, but it didn’t explain why they contain the spectrum of visible colors.

Isaac Newton did that. From 1664-1665, he worked on his famous experimentum crucis, which proved that refraction, per Ball, “splits white light into its component colors.” The design of the experiment is cool, and accessible on an intuitive level even for people who find the physics of light hopelessly abstract. Ball explains the experiment, which is depicted in the graphic below, like this (emphasis mine):

Newton's experimentum crucis, as he called it, had two components. First, he showed that the spectrum produced when a beam of sunlight shines through a slit into a darkened room and passes through a prism can be recombined into a shaft of white light by passing it through a focusing lens. Second, he used a mask to isolate just a single colour from the spectrum, and passed this alone through a second prism, finding that the colour did not change. Both experiments refute the idea that the light is somehow tinted by refraction during its passage through a prism or lens. And they show that the spectral colours are fundamental, irreducible to further subdivision.

Ball .jpg

Newton's experiment was incredibly elegant in design, but we know now that it probably didn't come off in practice as well as he claimed. Ball notes that even in modern lab conditions, it's quite difficult to isolate a single monochromatic band of light. This makes it likely that, when publishing his triumphant results, Newton augmented the imperfect outcome of the experiment with his theoretical understanding of what should have happened in that dark room.

You can read more about "Curiosity" here.

Image courtesy of the University of Chicago Press.

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About brainiac Brainiac is the daily blog of the Globe's Sunday Ideas section, covering news and delights from the worlds of art, science, literature, history, design, and more. You can follow us on Twitter @GlobeIdeas.
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Brainiac blogger Kevin Hartnett is a writer in Columbia, South Carolina. He can be reached here.

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Joshua Rothman is a graduate student and Teaching Fellow in the Harvard English department, and an Instructor in Public Policy at the Harvard Kennedy School of Government. He teaches novels and political writing.

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