

Title: On Colours (1665/1666)

Authors: Sir Isaac Newton

First Author’s Institution: Trinity College, Cambridge

Status: Open access via the Newton Project; parts later published in Philosophical Transactions (1671/1672)

You don’t need to make a great discovery in quarantine

Isaac Newton made some of his most important physics discoveries while in quarantine during the bubonic plague in the 1660s. As I write this from my own self-isolation during the COVID-19 crisis, I’ve been hearing many people cite this fact and proclaim, “You can use this time in quarantine to unlock your creative/intellectual potential too!”

Those people fail to mention that Newton did not have a packed schedule of Zoom meetings. Nor did he have to homeschool children, teach virtual courses, or keep up with the latest think-pieces about the pandemic on Twitter. He did not fall sick himself, or have to tend to sick loved ones. Also, he likely had others taking care of things like cooking and cleaning, being a well-off white dude in 1665.

This is all to say: please feel no pressure to pull a Newton and make the next big discovery during this time. But since everyone is name-dropping Newton, it seems like an appropriate time to dig into the physics he is purported to have discovered while isolating. I’d say that learning about what Newton learned during his quarantine counts as your productivity for the day.

Besides getting hit on the head by apples and inventing calculus, it turns out that Newton spent a lot of time in isolation playing with prisms. These formed the basis of his first paper on optics, and later his book Opticks, on the fundamental nature of light. As astronomy is based entirely on observing light (with the very recent exception of gravitational waves), this is as classic as it gets—Newton even coined the word “spectrum.” So here goes a quarantine-era edition of our series on Astrophysical Classics: Newton On Colours.

Newton plays with prisms

Thanks to the Newton Project, we can see the notes that Newton took during his time in isolation. Figure 1 shows a page from his notebook. While beautiful to look at, Newton could have written a little neater or used Evernote or something, so the project has helpfully provided a transcription. The section is titled On Colours and consists of 64 experiments and observations about light.

Figure 1: A page from Isaac Newton’s notebook while he was in quarantine in 1665-1666. Image via the Newton Project.

Back in the 1600s, there was no clear picture of light and color. People had observed that prisms, angled pieces of glass, turned white light into a colorful beam. One of the going hypotheses was that the prism “corrupted” the white light and produced colors.

Newton’s first great innovation was simply to shine the colorful light onto a sheet a bit farther from the prism. In his first prism experiment in Of Colours, he held the prism near a hole in the wall, and let the sunlight pass through the prism and land on a sheet of paper (Figure 2). He noted that “The colours should have beene in a round circle were all the rays alike refracted,” but instead he observed an oblong shape that appeared red on one end and blue on the other. This showed that prisms not only create colors but separate them from each other, and that refraction is directly related to the color of the light.

Figure 2: Newton’s first experiment with prisms, showing the refraction of light. Experiment 7 in Of Colours.

Newton took this a step farther in one of the final prism experiments of the notebook. He lined up three prisms and let light pass through all of them onto a wall (Figure 3). On the edges, he observed pure red and pure blue light. However, in between (or “betwixt,” as Newton says) these, the wall shone white. This meant that the colours “blended together” into white, so the light was not permanently corrupted by the prisms; rather, the colored light from different prisms could be recombined into white light. (In later experiments, he showed even more clearly that colored light can be recombined into white light by passing it through another prism.)

Figure 3: Newton shows, using three prisms, that colored light recombines to form white light. Experiment 46 in Of Colours.



The crucial experiment (and some less crucial ones)

These experiments were important precursors to what is known as Newton’s Experimentum Crucis, or crucial experiment. This was outlined in his first publication, “A Letter of Mr. Isaac Newton … containing his New Theory about Light and Colors” in Philosophical Transactions in 1671/1672 (years were weird back then). This paper, which made me appreciate AASTeX and matplotlib more than ever, aimed to show that color was a property of light itself.

In the experiment, shown in Figure 4 (diagram published later in Opticks), Newton let sunlight pass through a hole in the wall. He placed a prism ABC in front of the beam, and then blocked all but a small amount of the light that passed through this prism with board DE. After letting this light beam spread out over 12 feet, he again only let a small amount of light through board de, which he passed through a second prism, abc. The final beam hit the wall on the left.

Figure 4: Newton’s crucial experiment showing that refraction, and hence color, is a property of light itself. Diagram from Opticks, 1704.

In the diagram, Newton allows only the lower part of the initial beam, the redder part of the spectrum, to pass through the hole G. It ends up at point M on the far wall. Newton then rotates prism ABC. This shifted the part of the beam that fell on the hole G to be the bluer part, so now blue light passes through hole g onto prism abc. The final beam now hit point N! The initial color of light clearly determined the final refraction angle—independently of the thickness of the prisms, and of the angle of incidence. Refraction must be due to the properties of light itself. In Newton’s words, “Light consists of Rays differently refrangible.” He goes on to state that this is “not by any virtue of the glass, or other external cause, but from a predisposition, which every particular Ray hath to suffer a particular degree of Refraction.”

This experiment actually has quite a fraught history, regarding the original setup and what exactly it’s supposed to prove (see here and here for great deconstructions). But along with Newton’s related work, it revolutionized our understanding of light, all thanks to a toy prism he bought at a local fair.

Newton’s notes on color in quarantine end with him staring at the sun and pressing a blunt needle against the back of his eye to try to understand vision. (What is with astrophysicists and self-inflicted wounds while self-isolating?) So really, no pressure on the be-a-genius-in-quarantine thing.