why purple is on the end of the rainbow/color spectrum

Question

First of all, I don’t understand why indigo and violet are split… why not just call it purple? Ok, not my question.

Colors of the rainbow all make sense to me except purple.

Red – orange – yellow…makes sense that orange is between red and yellow, and it’s the combination of those two primary colors, nice transition

This follows with green, blue. Green is between yellow and blue.

Now… if purple aka indigo and violet are a combination of blue and red, how can it be on the opposite end of red (very different frequency) and outside the frequency range of blue? I would expect a secondary color’s frequency to be between the two primary colors that create it

In: 76

Answer 1

Extreme TLDR; The rainbow represents a single dimension of colors formed by single wavelengths of light; our eyes see color as a two-dimensional result of the balance between three receptors, commonly represented as a color wheel.

Look at something yellow on your monitor. Now, get real close, so you can see the individual dots. None of them are yellow. There are red dots and green dots… no yellow dots.

We have three kinds of color receptors in our eyes, roughly (ELI5) corresponding to red, green and blue/violet. Our sense of color comes from the balance between the strength of signal we get from each receptor. That means there are lots of ways to get any given color. We don’t see all the wavelengths of light independently, just the net effect on those three receptors.

In a rainbow, each color is a separate, single wavelength of light. Each wavelength (a “pure color,” if you will) stimulates the three receptors in a different proportion.

There are some proportions of receptor stimulation that can never happen from a single wavelength, but still can happen when more than one wavelength of light is present at the same time. Roughly, long wavelengths stimulate red, and short wavelengths stimulate blue/violet. Middle wavelengths stimulate green, and in between you get mixtures of red and green or green and blue/violet.

But what about red and blue/violet, without green? No *single* wavelength can do that, so you don’t see purples in the rainbow. But a combination of wavelengths can do that, which is how we get the purples. There is no *single* wavelength of light that can produce the sensation of, for example, magenta; but a combination of red and blue can do it.