This thought just hit me out of the blue today (no pun intended): If there are just three primary colors of light, then why isn’t a rainbow simply comprised of just those three colors? Have our teachers been lying to us all these years? Could it be that a strategically placed spectrum could break down the remaining (non-prime) colors into JUST the prime colors? How do you all you Newtons out there in the “Arched” Dope FIGure this? 
The 3 primary colours of light are a consequence of our vision rather than a feature of light itself, which simply varies in frequency ‘smoothly’ from what we call radiowaves to gamma rays. I think the seven identified colours of the rainbow are something to do with Newton’s mystical beliefs in the number.
Further to my previous post have a look here to see a graph of our eyes sensitivity to different wavelengths of light. The three peaks correspond to the 3 primary colours of light, and incidentally make colour TVs / displays much easier.
Out of three primary colors comes three secondary colors so we have 6 basic colors. Reason we see millions of colors is of luminosity and transparency.
Look at our computer graphics. We use three colors, Yellow-Magenta-Cyan to produce over 16 million colors (256x256x256) that we can see.
Primary colors are a cultural convention. Seeing millions of colors is a function of anatomy and neurology.
Our display technologies use RGB (red, green, blue) to synthesize colors additively. Cyan, magenta, and yellow are used with black (CMYK) in printing to synthesize colors subtractively.
https://cs.nyu.edu/courses/fall02/V22.0380-001/color_theory.htm
Here’s a simplified example: Let’s take the yellow light from the rainbow. We could take a singe wavelength of pure yellow light and shine it into our eye. Because the pure yellow activates both our green and red receptors at a certain percentage each we perceive the colour (correctly) as yellow.
Now let’s shine a mix of red and green light, in the same ratios, into our eye. Again, because the receptors are stimulated in the same way, we perceive the exact same yellow. But it isn’t.
The first is pure yellow. The second is a ‘fake’ yellow. To us they are identical. To a spectrometer it’s a single peak compared to a completely different double peak.
This is utterly wrong.
Light is a spectrum of electromagnetic radiation in specific wavelengths that happen to be visible to the human eye. The fact that we perceive certain colors more vibrantly is a quirk of how our eyes evolved a certain set of cone cells. There is no discrete point between one color and the next. The idea of there being only six discrete colors that mix together is just plain wrong. There are no limits to the number of colors in the world, but there are practical limits to what differences in wavelengths our eyes can detect.
The reason we talk about what my daughter calls the “M&M colors” is entirely arbitrary. Visit different cultures and you will find colors are defined differently. To directly answer OP, your teacher did in fact lie to you… In the sense that they often teach an over-simplified version to young children.
Also completely wrong.
The idea of blending colors out of three “primary” colors is useful when you are dealing with “subtractive” mediums like paint and crayons, and of course ink. Therefore, for practical and arbitrary reasons the printing industry arbitrarily decided that four colors of ink (Cyan, Magenta, Yellow, and Black) were “good enough.”
The fact that Cyan, Magenta, and Yellow are not objectively THE primary colors should be self-evident for two reasons:
(1) They are NOT the colors your TV uses. Your TV uses Red, Green, and Blue light because, again, the TV manufacturers decided it was “good enough.”
(2) Magenta is NOT a real color. Magenta is a combination of two specific wavelengths hitting your eye. Magenta does not exist on the visible spectrum. If you define a primary color as ‘a color we mix to make other colors’ then Magenta fails by even that definition.
I don’t know about anyone else, but if I were to categorise the colours of the rainbow from scratch, without knowing the cultural convention, I’d probably come up with “Red Orange Yellow Green Blue Purple”.
This “Indigo Violet” stuff is for the birds
Absolutely. Except you can safely slip cyan between green and blue. And purple is actually nearer to “technically violet” as the naming on UV (Ultra Violet) shows.
And as stated above, you’ll never get magenta in a rainbow. It’s a constructed colour, always a mix and this never a single wavelength…
…Look at your rainbow. Red ain’t next to Blue.
Right to a very good first approximation. But there is a sting in the tale. We see violet, and that is past pure blue. We should not perceive a spectral colour past the peak of blue sensitivity if you use a simple RGB mixing model. The sting is that there is a tiny blip in the red sensitivity in the far blue, and our brain actually does get a small mix of red with blue when seeing spectrally pure violet.
Part of the reason is that our eyes don’t have red green and blue sensors anyway. They have three sensor types that have very broad sensitivity, but with different slopes and break points across the spectrum. Subtract the signal of one from the others in different combinations and you can get the differences as signals in the three colour bands. However even this isn’t done, they eye processes the three different sensors into a luminance and two difference signals, and they is what is pumped back into the brain. (I was really impressed with some work done a while ago where the neurons in the back of the retina had their structure analysed and the logic of the interconnections reverse engineered, and out popped the sum and difference signals. Previously the nature of the signalling had been painstakingly derived by working through the precise colour sensitivity the eye brain system exhibits, and fitting the model to the way we see both spectrally pure and mixed colours.)
A rainbow does not have seven stripes. It does not have six stripes.
It has an infinite number of stripes, gradually shifting from one hue to another.
How many categories we choose to assign, and where the dividing line is, between one category and another, is entirely arbitrary.
If we wanted, we could pick a band between yellow and green, and label it chartreuse. If we wanted, we could pick a band between yellow and chartreuse, and name it something else. We can choose whatever boundaries and labels suit our purposes at the moment.
I love this. Gap in my knowledge filled. Thank you.
There is also a large cultural aspect to the perception/description of color. The Greek poet and philosopher Xenophanes, for instance, describes the rainbow as follows:
[
](Xenophanes of Colophon - World History Encyclopedia)
(He was writing to dispel the notion that the rainbow was a manifestation of the goddess Iris, considering all such things to be simply poorly understood natural phenomena.) So he did consider the rainbow to have three colors.
We did that a couple centuries ago. Wiki on ‘orange’:
I, for one, am hard pressed to find the indigo between the blue and violet in a rainbow.
Absolutely correct. I wrote an article about Indigo, which forms a chapter in my book How the Ray Gun Got Its Zap!
There are almost infinitely many colors in a spectrum. We do, indeed, have tristimulus vision, meaning that we have three degrees of color sensitivity, but we can use those to visualize a vast array of colors. The thing is that many colors that have very different spectral composition look exactly the same to us. It’s why we can get away with representing an entire spectrum using three colors in printing, or three different phosphors on a TV screen, and so forth.
People have disagreed about how many colors there are throughout history and cultures. Our present claim to their being seven colors comes, not from ancient history and mysticism (as is frequently claimed), but from Sir Isaac Newton.
When Newton started writing his treatise on Optics, he imagined that there were five colors – Red, Yellow, Green, Blue, and Violet. Some passages in the book still retain this Five Color scheme. Other colors were just mixtures of these.
But he changed his mind, for some reason. I suspect it was his playing around with the interference phenomenon called "Newton’s Rings’ (although he was not the first to discover it). If you separate white sunlight using a prism, you get a single red-to-violet spectrum. But if you place a hemispherical piece of glass rounded side down on an absolutely flat piece you will see concentric rings of color, centered at the poit where the pieces of glass touch. And these rings follow the same order as the prism-sepoarated colors – red, yellow, green, blue, violet (to use his 5-coloor scheme). But then they repeat – RYGBV-RYGBV - RYGBV. It’s very much like the way keys on a piano or harpsichord repeat themselves, the way musical notes rise from octave to octave.
There’s a characteristic length associated with the colrs, as Newton found when he calculated the sizes of the gaps between the two piece of glass. And the distances corresponding to “red” are integral multiples of each other, as are the distances corresponding to each of the other colors. To another mind, this might be seen as evidenced of the wavelength theory of light. But Newton was committed to the corpuscular theory of light that had it made of particles.* He did, however, make the connection with the tonal scale, and concluded that there ought to be seven colors in a spectrum, corresponding to the seven notes in an octave (if you ignore the repeated “C” or “do”) He compared the relative lengths associated with his colors and found that they agreed nicely with the lengths of strings needed to make those notes, with Red, Yellow, Gren, Blue, and Violet fitting very nicely. But there was a gap between Red and Yellow, and another between Blue and Violet. Orange was the obvious choice between Red and Yellow, and its length fit perfectly. But he needed a color between Blue and Violet. So he INVENTED “Indigo”. The only reason it exists is to fill that gap in the color “scale”. reportedly, newton used to try to persuade his friends that they could see an obvious color difference between Blue and Indigo.
I can’t see it myself. Indigo is the plant they used to use to dye Blue Jeans. To me, it looks blue. Most people, I suspect, don’t see the difference. The Universal Code for Resistors follows a rainbow order, but leaves out Indigo – Brown Black Red Orange Yellow Green Blue Violet. The “Rainbow Flag”** you see most commonly either has only six colors (leaving out indigo) or else has a light blue or cyan in place of indigo.
By the way – rainbows themselves don’t always have seven, or even six obvious colors. The relative widths and appearance of the colors depends upon the sizes of the raindrops. If the raindrops are too small, diffraction from the small size fights against the separation refraction produces, making the colors blend together. In the most extreme cases of “mistbows”, the bow is completely white, or may even have the orders reversed. M. Minnaert, in his classic book The Nature of Light and Color in the Open Air gives a handy chart that lets you determine the size of the raindrops from the observed widths and appearance of colors.
*Over a century later, when Thomas Young was championing the wave nature of light, he made the first measurement of the wavelengths using a fortuitous diffraction grating, and compared his values to Newton’s.
** neither of these are the original “Diversity” Rainbow Flag, which actually has the colors in a slightly different order.
I’ve never understood the difference between violet and indigo. I know what those words are supposed to mean, but when I look at colors it’s all just “purple” as far as I’m concerned.
I also spent time in Korea and I learned they define the colors differently. Korean’s definitions of blue and green are slightly different from Americans.
I’ve always wondered if the “blue” Newton used was supposed to be more of a sky blue/cyan, and the indigo more of what we think of as regular Crayola blue. That would make a lot more sense to me, as when I look at a rainbow, there’s (to me) a clear cyan-sky-blue as green transitions into deep blue, but not an obvious delineation between blue and violet, such that would necessitate naming another color (in this case, indigo) in between them.
You’ll also sometimes (again, due to diffraction effects) see extra (or “supernumary”) bands below the violet. To my eye, they stand out most prominently as alternating bands of green and purple.
And even without diffractive effects, the purely refractive effects will also result in each band having some thickness, as will the nonzero angular size of the Sun. That is to say, if you were to look at a large-drop (i.e., negligible diffraction) rainbow with an extremely high resolution telescope, so you’re only looking at a tiny width of the rainbow, and fed that through a spectroscope, you’d still see some spread of wavelengths, just with the peak in different places.
Supernumerary rainbows are further evidence of the wave nature of light, and the diffractive nature of the rainbow itself – you can’t explain them if you assume that rainbows are a purely refractive phenomenon, and Thomas Young, ever on the lookout for evidence of the wave nature of light, eagerly seized on them as powerful evidence for his case. His math wasn’t quite up to handling it, though, and it remained for George Biddell airy, director of the Greenwich Observatory in the early 19th century, to derive the Rainbow Integral and finally put the diffractive rainbow on a solid mathematical footing.
You only see the supernumerary rainbow if a.) the drops are all significantly larger than the wavelength of light; and b.) if the raindrops are all pretty nearly the same size. That’s why they’re comparatively rare. I’ve seen them in nature, though. as for why they’re alternative pinkish-purple and aqua-green 9rather than repeating the spectrum, as with Newton’s Rings0, it’s because they’re the result of Multiple Order white Light Interference. the overlapping out-of-synch orders produce basically pink and green light. You see the same effect in oil films and soap bubbles. I have an upcoming article on this (Entitled “Preppy Optics”) in Optics and Photonics News.
Indigo is a particular dye. Purple is a different particular dye. IMO they look quite different. That is without regard to names (and colour names differ greatly from language to language as you point out); of course violet is also named after a plant. So is fuchsia. Newton did not make up all the names himself.
Furthermore, in colour theory, there is an entire “line of purples” that are not in the rainbow spectrum; you get them by mixing red and violet or red and blue in various proportions.
As you noted, the names are all cultural/linguistic/artistic convention, but the (infinitely many) colors are quite real even when you call them all “shades of purple”.