May have worded this poorly, but when you look at the color spectrum, it appears (to me, my thinking may be flawed) to be the primary colors red, yellow, and blue and their intermediates. Red to yellow with orange in the middle, yellow to blue with green in the middle, and blue to what would be red, with purple in the middle. Except there is no red at the end of the natural spectrum, just at the beginning. So is it just a nice coincidence that it wraps around perfectly?
In: Physics
The sensation of colour is generated in our brains, as a way to interpret the signals from the cone cells in our eyes. The cone cells come in three different types, sensitive to different wavelength ranges of light. When light of a certain wavelength hits the retina, it stimulates the three different types of cone cells by different amounts. Those different amounts of stimulation are sent to your brain, where you brain assigns a “colour” to the combined signal.
As it happens, light of the shortest wavelengths we can detect, around 400 nanometres, our brain interprets as “violet”, while light of the longest detectable wavelengths (around 750 nm), our brain interprets as “red”. The fact that red and violet seem to be similar colours that can blend into one another comes from the fact that our brain also needs a way to interpret a *mixture* of wavelengths, some around 400nm plus some around 750nm, with no intermediate wavelengths.
If you have a mixture of 400nm plus 750nm light, your brain gets signals from the short wavelength and long wavelength sensitive cone cells, but *none* from the middle wavelength cone cells. Your brain interprets this as magenta – a colour “in between” violet and red. Magenta cannot be generated by a single wavelength of light – it’s not in the rainbow.
Your brain needs to be able to move continuously from violet to red via this magenta sensation, to account for different ratios of short+long wavelength light. So your brain considers this to be a continuum of colours fading smoothly into one another.
**In summary:** No, it’s not a coincidence. But it’s also not an inherent “fact of nature” that 400nm light looks “similar” to 650nm light. It’s a result of the architecture of our eyes and brain.
For example, birds have extra cone cells sensitive to ultraviolet light. To them, 400nm and 750nm light most likely look very different, and the wrapping around in “colour” only occurs for the wavelengths around 250nm (in the UV where we can’t see) and 750nm.
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