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I have a knot about the "secondary" colours and its spectra. So If I look the spectra of white light, I can see that yellow, which is between R and G, has some wavelength. If I do an addition of light of Red and Green, I get yellow. Ok. Now If I do an addition of light of Red and Blue, I get Magenta. But how come that Magenta is only a perception and has no wavelength, as Yellow for example ?

I understand that some colours physically do not exist, but I dont understand why yellow exists as a real physical wavelength but not magenta.

Can someone help please

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Human color vision depends on three different types of light-sensitive cell (a.k.a., "cone cells") which are called, "L," "M," and "S." And, each of them has a different response to various wavelengths of light.

Plot showing relative spectral response of L, M, and S cone cells

(Image copied from: https://commons.wikimedia.org/wiki/File:Cone-fundamentals-with-srgb-spectrum.svg)

Human color vision—how your brain interprets the signals from those cells—is complicated, and maybe is not yet completely understood; but in order for you to perceive the color we call "magenta," the light source must cause your L cells to respond more strongly than your M cells, and the same source must also elicit a significant response from your S cells.

It should be obvious from looking at the diagram that no single wavelength of light can meet both of those requirements.

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    $\begingroup$ I think what's missing in this answer (but is sketched by RC_23) to clear up OP's incomprehension is what happened with both type of yellow: roughly, the monochromatic yellow reacts with the same intensity with M cones and L cones, and not at all with S cones ; but the exact same thing happened with a combination of monochromatic red and green. So our brain, relying exclusively on the answer of those threes cones, can't know any better and interprets it the same way. $\endgroup$
    – Blackhole
    May 17, 2023 at 23:25
  • $\begingroup$ @Blackhole, I kind of left that out on purpose. Didn't want to end up writing a wall of text. $\endgroup$ May 18, 2023 at 2:00
  • $\begingroup$ @Astro_medi, if you want to know more about different ways of making "yellow," try reading the first couple of paragraphs of this article: en.wikipedia.org/wiki/Metamerism_(color) $\endgroup$ May 18, 2023 at 2:01
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The way our brain interprets colours doesn't map super closely to how the physical spectrum is laid out.

There are numerous ways to display the spectrum in ways that make more sense. One of them is the 1931 colour space:

enter image description here

(From Wikipedia)

All of the numbers along the edge correspond to a single, pure wavelength. Adding wavelengths together result in one of the colours in between, depending on the balance of each wavelength intensity. You can see that if you have an LED right at 560nm (green) and another LED right at 580nm (orange), then combined, they will appear yellow. Same thing with 560nm and a smaller amount of 600nm(red).

Side note, you will also get yellow if you have a single LED at 570nm. This is distinct from the previously discussed, but your brain won't be able to tell the difference - This is called metamerism.

Following this exercise through, if you mix a combination of 540, 600, and 480nm together, you'll hit that white patch in the middle. This is the basis of how your monitor works, using only a gamut of RGB.

Given this mental model, it's easy enough to see how you can achieve yellow using green and red, and why blue 460nm and red 600nm eventually reach magenta shades.

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    $\begingroup$ Note that the xy chromaticity chart given here is not exact in terms of colors: all the colors here are desaturated to fit into the sRGB gamut (otherwise it would only be possible to display a triangle around the D65 point, with other points being unrepresentable). $\endgroup$
    – Ruslan
    May 17, 2023 at 6:52
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The "yellow" you get from a single wavelength like a sodium lamp, and the "yellow" you get from mixing red and green lights only look the same because of the way your retina and brain process light. There is really no physical reason inherent to the nature of light why that should be the case. In a creature with different color receptors, those could represent two totally different colors.

Colors not on the monochromatic spectrum, such as Magenta and White are essentially "invented" by our brains in order to interpret certain mixtures of Red, Green, and Blue.

Here's a good video of this topic

https://youtu.be/-E6dxybpUvo

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  • $\begingroup$ "only look the same because of the way your brain processes light" - This is wrong. The cones in our eyes only have three degrees of freedom, so it's impossible for our eyes to uniquely identify all possible spectra and metamers are a mathematical certainty. No matter how your brain processes the signals, those two spectra will always look identical because the signals coming from your eyes are identical. $\endgroup$ May 16, 2023 at 23:24
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    $\begingroup$ I think you are proving my point. If we had a different set of receptors, the yellows might not look the same $\endgroup$
    – RC_23
    May 17, 2023 at 3:50
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    $\begingroup$ @RC_23 perhaps the answer could be improved by replacing "brain" with "retina & brain" as BlueRaja does make a good point that some of the issues are introduced in the retina before reaching the brain $\endgroup$
    – Tristan
    May 17, 2023 at 15:13
  • $\begingroup$ Not a bad idea. $\endgroup$
    – RC_23
    May 17, 2023 at 18:04

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