Basically what the title asks.
All the other colors of the rainbow are either primary or secondary. Violet is a shade of purple and (as I’ve always understood it) indigo is somewhere between navy and purple.
In: Other
“Purple” isn’t a wavelength of light. It’s a *harmonic*.
We have specialized cells in our eyes to detect light with peaks at certain wavelengths (color), but only 3 colors:
– red
– green
– blue
As you can see, there is no violet detector. We detect “in between” colors like yellow being in between green and red when both sets of red and green cells are stimulated.
Red is light with a space between its peaks of at most about 750 nanometers. At the other end of spectrum of what we detect is blue at about 450 nm at the shortest.
If you know much about music, a harmonic is when you can fit two waves in the space of one. This means that there would be a peak in the larger number range every two periods.
Violet is about 380 nm. 380 x 2 ≈ 750 — so it sets off the red detector **and** the nearby 450 nm blue detector.
Since we usually see these 2 receptor colors as “in between” the primary colors, we perceive it as a mix of red and blue as though they somehow mixed to make a color way outside their wavelength. But they don’t. We call that imaginary mix of red and blue purple.
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