Is the "indigo" stripe of the rainbow more blue or violet?

The traditional description of the colors of the rainbow (since Isaac Newton established it) is that there are seven of them: red, orange, yellow, green, blue, indigo and violet. “Indigo” is one that has kind of annoyed me since childhood: the other six color descriptors are all hues (i.e. base colors), whereas indigo is a nuance, thus only a shade of a color. By that standard there should not be seven, but six “main” colors in the rainbow; otherwise, the “blue” stripe would be better described as “turquoise”, “teal” or “cyan” (Newton, who has been described as the last alchemist and the first modern scientist, assigned 7 colors to the rainbow because 7 is a “magic number” and corresponds, for example, to the 7 notes of the major scale; read: unscientifically). But let’s talk a bit more about that contentuous stripe. When I hear “indigo”, I understand that to mean a very dark unequivocal blue. It’s a natural pigment from which blue dye was traditionally made and I take that shade to mean the color of a clear night sky, or of dark wash jeans. Yet in childhood I somehow got the impression that it was more of a violet. This impression may have been created by a realistic painting of a rainbow that I saw that I mistook for a photograph. I even once debated my second-grade teacher on the matter! But looking at a rainbow with the naked eye, the stripes are too thin to make the exact color out clearly.

In actual fact, is there any substantial red content in the “indigo” stripe, which would make it a shade of violet/purple (or at least “blue-violet”, I.E. the tertiary color), or is it really a more or less pure blue? Has this even been determined scientifically, or is it purely a matter of perception? Is it more of a gradient? I.E., is the “indigo” stripe actually part blue and part violet?

The red pigment in the eye has a small amount of reaction to deep blue/violet wavelengths. So they look purplish, even though they don’t have any actual red color.

The short answer to your question is no; there is no meaningful “red” content of the light in the “indigo” portion of the spectrum. A prism disperses light with different wavelengths at distinct angles; all of the light with wavelength 700 nanometers (red) goes along one path while all of the light with wavelength 450 nanometers (indigo) goes along a different path. So the 450-nm light at one end of the spectrum doesn’t contain any mixture of light with 700 nm (or vice versa.) Note that this same argument applies to the violet light at the very far end of the spectrum as well (not just the indigo light).

That said, the details of human color vision may cause light at the blue/violet end of the spectrum to be interpreted as red, as noted by dtilque above.

Your and dtilque’s comments prompted me to google the origin of the violet (I.E. where does it come from if not from a wavelength containing red) and I found this handy video: https://www.smithsonianmag.com/smart-news/rainbows-dont-include-purple-light-so-why-do-they-sometimes-seem-180953190/

So it’s created by “supernumerary rings”, I. E. a less conspicuous rainbow within the rainbow and THEIR red content.

Now it makes sense that the “indigo”, at least where not covered by a supernumerary ring, is indeed a dark blue.

Nothing in Newton’s writing suggests that he chose it because it was somehow “magic”. But he does state that he chose it by analogy with musical tones. That sort of makes sense when you see how the spectra start to repeat in sequential “orders”, just as octaves on a piano follow one another. Newton would have seen this i several phenomena, most notably in what have been called “Newton’s rings” in his honor.

Before he decided that colors in the spectrum came in groups of seven, he had thought that there were four distinct colors (some parts of his book Opticks still contain discussion of the four colors, although he revised it to seven in most places.) He based his divisions on the proportionate variations in the musical scale. Although Newton did not hold with the wavelength theory of light, he did know (from his work on Newton’s Rings) that there was a characteristic length associated with each color. So it wasn’t just the number of colors that came from the analogy to musical scales – it was also their spacing.

In addition to red, yellow, blue, and violet, from his original four, he now had gaps to fill in with the other three, their positions dictated by the relative spaces in the musical scale. Orange and Green were pretty obvious, but the color between blue and violet was more difficult. Even Newton’s friends had trouble discerning a different color in there, and he cajoled people into agreeing with his contention that there was a different color in there, because the theory said there had to be. Newton decided to call this color “indigo”, not because it really was the color, or because that color was the perfect exemplar, but because he needed a name, and that color looked, to him, pretty much like what he wanted.
Indigo is the dye used to color blue jeans, and I would unstintingly call it “blue”, but the name has persisted. I suspect part of the reason is that people like the idea of seven colors, and it also gives Roy G. Biv a last name that, if not believable, is still pronounceable.

I also note, however, that most of the “rainbow” flags only have six colors, leaving out Indigo.

I also note that the Uniform Color code used for resistors, which follows the spectrum except at the ends, also dispenses with indigo. You want . easily distinguishable colors on your resistors, and I have no doubt that using indigo as a color in the code would have caused no end of confusion.

finally, I have to observe that there’s not really any reason to divide either colors or notes up into octaves or seven colors. Both scales are arguably continuous ones. You can easily interpolate colors or notes by using frequencies between the standard values. In fact, the "“standard” values of the musical notes depend upon the temperament you are using (and brings up the interesting question of what Temperament Newton was using when he calculated his colors. I answered this in an article I wrote a couple of years ago. But the difference aren’t really large enough to affect the way the colors appear.)

Indigo is the name of a dye, not the name of a color. The color of indigo dye is blue. As evidence for this, I present the most common object dyed with indigo, whose name includes the adjective “blue”.

You’ve only got to google indigo to see that there are quite a few people who equate it to a deep blue-purple colour, which is technically not right - natural indigo dye is deep, fairly pure blue - Denim blue; not as dark as navy blue, but the same sort of hue, but there is enough general confusion that many people think indigo means ‘dark blue-purple’
For example, this organisation called The Indigo Group, whose banner does not actually contain any indigo colour at all.

As others have said, the stripes in the rainbow aren’t really there - they are a combination of artificial/societal construct, influenced somewhat by the way our own eyes perceive light.
What I mean is that there isn’t a ‘band’ of the same colour that has a width, bordered by an abrupt change to another band that is also the same colour across its width - it just isn’t like that.

So insofar as there is a band called ‘indigo’ in the rainbow, it is a gradient that goes from blue at one end, and ‘violet’ at the other - violet not containing any wavelengths that we would identify as ‘red’, but accidentally stimulating our red colour perception a bit, so as to appear so.

It seems pretty clear based on Newton’s color wheel that what Newton called blue is closer to what we now call cyan, and that his indigo is closer to what ww think of as a pure blue.

It makes sense: we refer to the sky itself as blue, when it’s closer to cyan than pure blue.

As for why we see violet as having some red, I’m seeing renewed claims that it’s because there is a slight hump in the response of our red cones in the violet area. Though others say it’s not the cones themselves but how color is processed in the brain. Either way, the result is that we can simulate violet with purple–mixing blue and a small amount of red light.

But pure spectral violet does not contain any red whatsoever. It’s just a shorter wavelength than blue, but longer than ultraviolet and thus still visible.

Newton would disagree with you. He clearly meant for “indigo” to be the name of the color between blue and violet. The color of indigo dye, I agree, is what I would certainly call “blue”. But Newton’s “indigo”, as I state above, is not necessarily associated with the color of the dye produced from the indigo plant, but was probably his closest natural color he thought approximated the color he wanted to give a name to.

Somewhat leading on from CalMeacham’s, post I don’t think it’s really an answerable question. Given that:

  1. Indigo, just like most colors, really covers a range. In fact, indigo is worse than some because human eyes don’t differentiate shades between blue and violet very well.
    Probably “indigo dye” is what most think of as “true” indigo, but even that covers a range, and why would we be so specific in terms of what counts as indigo, when the chunk of the rainbow we’re calling “yellow”, say, is quite wide?

  2. Of course, the shades in a rainbow are really a continuous spectrum with no reason to count them as any specific number of colors. BTW If we could see UV and IR, we’d see a wider rainbow extending into those parts of the spectrum.

  3. The OP mentioned “real” colors, but it’s not even that simple to say what, say, the primary colors are, because of the details of human vision. Yes we have 3 types of cone in the eye, but their sensitivity spectra don’t map perfectly to the ranges of named colors like “red”, and they overlap a lot. Further, there is a “post-processing” that happens in the neurons of the eye and the brain that do things like, for example, map different inputs to the same subjective shade.
    These nuances matter when we want to talk about how many “real” colors are in the full visible spectrum.

I’m resurrecting this thread because today I saw an absolutely magnificent rainbow. It was around 8 PM and the sun was setting but there was still enough light, and the rainbow stretched wide in the East as a more or less complete semicircle in the sky. There was even a second flipped rainbow to its right. I looked closely at the stripes that we were discussing above, and I noticed that this particular rainbow had very little of an identifiable “blue” hue in it. The stripe that is normally called “blue” seemed largely merged with the green (and indeed, as said above, the “blue” stripe is more of a cyan or teal, thus with evident green content), and the violet stripe was very wide (I could tell why - there was a clear supernumerary rainbow underneath). The “indigo” stripe was very narrow and to my eye looked more purple than blue (as with the violet, I strongly suspect that this was again the doing of the red stripe of the supernumerary rainbow below it).

If you’ve got a rainbow that is being created by reddish sunset light, there might not be much blue to see ( the blue light it was scattered away before it went past you on the way toward the rainbow)

Not all rainbows look alike. The appearance of the rainbow and the presence or absence of colors and their relative width depends upon the size of the raindrops making up the rainbow. In his seminal book The Nature of Light and Color in the Open Air , M. Minnaert provided a nifty chart that told what rainbows looked like as caused by different color drops. You can also see plots of the effect on various websites, nowadays rendered in full color

http://www.philiplaven.com/p5.html

Figure 8-11 here:
https://www.usna.edu/Users/oceano/raylee/RainbowBridge/Chapter_8.html

The reason that this happens is that you only se really well-separated colors when the raindrops are significantly larger than the wavelength of light. (that’s also when you see the supernumerary bows). As the drops get smaller, the effects of diffraction start to come in. In a way, the drops act like the slits in an interference experiment, and the smaller they get, the more each band spreads out, so that the different colors overlap more and more. The two effects of prismatic separation and diffractive spreading finally balance each other when the sizes of the drops are very close to the wavelength of light. Then you have a bow, but it’s completely white. This is called a “mistbow”.

So there are sizes for which there is very little blue, and there are other variations, as well. In fact, there’s no reason for all drops to be the same size. Even if conditions favor one drop size in a particular region, they might be different in another cloudburst along your line of vision, and rainbows are formed by scattering from all droplets along that approximately 42 degree line from the antisolar point, no matter how far away. So your actual spectrum might be due o combinations of different sized drops on that chart. And, as noted above, you might be viewing a rainbow near sunset, when more blue is scattered out. Or you might, as you say, be seeing the effects of supernumerary bows.

Complex things, rainbows.