Why Does a Rainbow Have Seven Colors? Newton, Music, and the Cultural Definition of Color

In 1665, as the plague spread through Cambridge and the university shut its doors, a young Isaac Newton, still in his early twenties, retreated to his family home in Woolsthorpe and got his hands on a glass prism. In a darkened room, he cut a small hole in the window shutter to let in a single beam of sunlight, then passed that beam through the prism, casting a long band of color across the wall opposite.^[1] But on that wall, the band of color Newton actually saw had no sharp edges at all. The shift from red to orange, from green to blue, blurred so smoothly that no eye could pin down exactly where one color ended and the next began. And yet today, textbooks around the world teach that a rainbow has precisely seven colors: red, orange, yellow, green, blue, indigo, and violet. If the boundaries between colors are invisible, how did the number seven ever become so exact?

From Five Colors to Seven

Newton did not see seven colors from the start. In his early experiments, like the chemist Robert Boyle before him, he identified and recorded only five colors in the spectrum — red, yellow, green, blue, and violet.^[2] It was only through later, repeated experiments that he added orange and indigo, bringing the total to seven.^[2]

Why go from five to seven? The answer is hinted at in the original text of Newton’s Opticks, published in 1704. Describing how he divided the widths of the spectrum’s colors into specific proportions, he wrote that those proportions matched “the intervals of a musical octave, that is, the ratios between the tonic and the second, the minor third, the fourth, the fifth, the major sixth, the minor seventh, and the octave.”^[3] Elsewhere he described dividing a string so that it sounded the eight notes “sol (D), la (E), fa (F), sol (G), la (A), mi (B), fa ©, sol (D),” with the seven intervals between those notes matching the widths of the seven colors.^[3] The syllables sol, la, fa are the older solmization names Newton actually used, not today’s do-re-mi-fa-sol-la-ti; the letters in parentheses are the modern note names. Notice that this scale begins on D, not C. In other words, Newton did not divide the spectrum according to what the eye actually saw. Instead, he mapped it onto the Western diatonic scale — the seven-note system that splits an octave into do-re-mi-fa-sol-la-ti — and slotted orange and indigo in at exactly the points where the half-steps fall, the way they do between fa and mi, or between ti and do.^[2]

Why He Tried to Fuse Light and Sound

What makes this all the more striking is that Newton did not present this scheme as a result of his optical experiments — he framed it as an analogy between light and sound. In Opticks itself, he cautiously wrote that this seven-color spectrum “might suggest” a correspondence between the harmony of sound and the harmony of color.^[3] In other words, he did not measure the wavelengths of light with enough precision to discover seven natural divisions. He simply took a seven-note system that already existed in music and applied it to color.

Physicist and pianist Peter Pesic has called this decision “a very strange and interesting thing to do,” noting that “there’s no experimental basis for it whatsoever. Newton was really just superimposing something onto the color spectrum through an analogy with music.”^[4] Among the educated circles of 17th-century Europe in which Newton moved, there was a deep-rooted belief that the order of the universe was built on the number seven. Seven planets were known at the time (counting the sun and moon), a week had seven days, and alchemy recognized seven principal metals — the number seven itself was widely held to symbolize cosmic harmony.^[5] Newton is remembered as a founder of modern science, but he also poured enormous amounts of time into alchemy and biblical chronology. His attempt to unify color and sound into a single system was the product of two Newtons overlapping: the experimental scientist, and the seeker of the universe’s hidden order.

One caveat is worth flagging when telling this famous story. It’s often dramatized as though Newton were staring at a rainbow and suddenly decided, on the spot, to match it to music. But the actual text of Opticks is far more restrained than that popular version. Newton merely noted that the analogy between color and sound “might suggest” something — he never claimed it as an absolute truth about the universe.^[3] The more accurate reading is that the dramatic version circulating today was gradually exaggerated and simplified as it passed through later histories of science and popular accounts.

The Spectrum Never Had Boundaries to Begin With

This is the point worth pausing on, because it’s the thread running through this entire piece. A rainbow is, physically, a continuous spectrum. The wavelengths of light form an unbroken range of values, and between any two wavelengths there is always a finer wavelength in between.^[6] So there is no physical boundary line in a rainbow where you could say “red ends here, orange begins here” — no such line has ever existed. The number of colors isn’t a matter of observation; it’s a matter of how many linguistic categories humans choose to carve out of a continuous band of light, and what we choose to call them.

Seen this way, the question “does a rainbow really have seven colors?” doesn’t even make sense as a question. A rainbow could just as easily be described with five colors, six colors, or broken down into twelve. The very fact that Newton started with five and later stretched it to seven shows how much the count depends on the observer’s own scheme of classification. That’s exactly why the debate over how many colors a rainbow has never really settled, from Newton’s time to now.

The Never-Ending Argument Over Indigo

Of the seven colors, none is more contested than indigo. Plenty of people struggle to tell indigo and violet apart at all. In his early notes, Newton himself wrote that “the original primary colors are red, yellow, green, blue, and a violet-purple, to which are added orange, indigo, and the countless intermediate gradations between them.”^[7] He placed indigo between blue and violet — right at the spot that corresponds to a musical half-step.

Modern color scientists are considerably more skeptical. Physicist Gary Waldman has pointed out that “what Newton called indigo is close to what we would simply call blue today, and what Newton called blue is closer to what we’d now call cyan.”^[7] Author Isaac Asimov made a similar point, arguing that indigo “just looks like a shade of dark blue” and that there isn’t much basis for treating it as a separate color at all.^[7] In practice, many modern spectroscopists do split the spectrum around 450 nanometers, but they don’t attach the name “indigo” to that boundary.^[7] Several researchers have made the same observation: the human eye’s own color-discrimination ability simply isn’t well suited to perceiving indigo as a category clearly distinct from blue or violet.

So why has indigo survived in textbooks and popular belief all this time? Mostly, it’s inertia. The mnemonic “Roy G. Biv” has been embedded in educational materials and popular culture for centuries now, and once a framework like that takes root that deeply, it’s hard to dislodge even after the science underneath it wobbles. Few examples make the point better than the indigo debate: what decides how many colors we count isn’t optics — it’s convention and schooling.

Aristotle Saw Only Three Colors in the Rainbow

About two thousand years before Newton, Aristotle, in ancient Greece, classified the rainbow in an entirely different way. In his Meteorology, he identified red, green, and violet as the rainbow’s fundamental colors — the three colors, he argued, that no pigment mixture could ever produce. He acknowledged that yellow sometimes appears in a rainbow, but he explained it away not as a fourth color, but as an optical illusion: when red sits next to green, the contrast makes the eye perceive something whiter, and therefore yellow.^[8] Aristotle’s theory of color held authority across European intellectual life for close to two thousand years, until Newton’s experimental optics finally displaced it.^[9]

The gap between Aristotle and Newton isn’t simply a difference in how carefully each man looked. Both were looking at exactly the same physical phenomenon — the same rainbow. But Aristotle sorted it into three colors within the framework of his theory of the four elements and light-dark contrast, while Newton sorted the very same phenomenon into seven colors within an entirely different framework: musical harmony. The object of observation was identical. What differed was the mental framework used to divide it up.

Cultures Count Rainbow Colors Differently

This kind of divergence isn’t limited to ancient Greece versus early modern England. Even today, cultures around the world count the colors of a rainbow differently. In some regions, children are taught that a rainbow has five colors; in others, six or more.^[10] One influential theory that explains this gap is the “basic color terms” theory proposed by linguists Brent Berlin and Paul Kay in 1969. Surveying languages from around the world, the two researchers found that the number of basic color words varies from language to language, and that the order in which those words tend to appear follows a fairly consistent pattern. Languages with only two color terms typically distinguish light from dark; red tends to be added next, followed by green or yellow, and only later does blue enter the picture.^[11]

A field study among Namibia’s Himba people put this theory to a real-world test. A research team led by Professor Jules Davidoff at Goldsmiths, University of London, found that speakers of Himba — a language with an unusually rich vocabulary for subtle shades of green — could easily pick out fine differences between shades of green that English speakers would barely be able to distinguish at all.^[12] That said, the way this research reached the public wasn’t free of exaggeration. One documentary dramatized the findings by claiming that the Himba “couldn’t tell blue and green apart at all” — a portrayal that linguists have since pointed out was a piece of staged television editing, not something the researchers themselves ever claimed.^[13] What the actual research showed was not that the Himba are unable to “see” blue and green, but that their language tends to lump blue and green into a single category while subdividing greens more finely — which meant it simply took them a bit longer to separate the two colors in the experiment.^[13] In other words, it isn’t that they see color differently. It’s that their language draws the dividing lines differently.

The Japanese word ao (青) offers a similar case. For a long stretch of its history, Japanese described the world’s colors using only four basic terms — red, black, white, and ao — and ao covered what we would now call both blue and green.^[14] A separate word for green, midori (緑), only began to establish itself during the Heian period (794–1185), and even after that, ao continued to be used for green tones for a long time. Traces of this survive to this day. When Japan installed its first traffic lights in 1930, newspapers referred to the green light as ao shingo (青信号, “blue signal”), and that term was written into law in 1947.^[15] After repeated criticism that this clashed with international traffic standards, the Japanese government settled on a compromise in 1973: traffic signals would still use green that met international standards, but manufacturers would produce it in a shade tinted as blue as possible.^[15] It’s a striking example of just how long the words a language uses to divide up color can shape — and hold onto — the way people actually see it.

In the End, It Wasn’t the Eye That Divided the Colors

The wavelengths of light that make up a rainbow have not changed at all — not in Newton’s day, not in Aristotle’s, and not now. What has changed is the human habit of slicing that continuous light into a set number of bins and naming each one. Aristotle drew three bins. Newton drew seven, tuned to the musical scale. The Himba drew theirs by subdividing the green end of the spectrum. Old Japanese drew its lines by folding blue and green into one. The “ROYGBIV” system that survives in textbooks worldwide today isn’t a physical truth about light — it’s closer to a convention that happened to stick, born from one 17th-century scientist’s attempt to find the universe’s harmony in music. The rainbow has always hung there in the sky as a single, unbroken band of light. How many colors to divide it into was always up to whoever happened to be looking.

That habit of taking Newton’s seven colors on faith is something English-speaking students learn especially early, and in more than one version. Generations of American schoolchildren have memorized the acronym “Roy G. Biv,” while many British and Commonwealth students were instead taught the sentence “Richard Of York Gave Battle In Vain” — two different mnemonics built on opposite sides of the Atlantic, yet both encoding the exact same seven-color assumption Newton left behind.^[2] Neither one pauses to ask whether indigo deserves its own letter at all; both simply hand down the convention as though it were as fixed as the wavelengths themselves. Newton’s musical analogy, in other words, didn’t just reshape how physicists described light — it reshaped how English-speaking schoolchildren, on both sides of the ocean, have been taught to recite it ever since.