Is it just a coincidence that the color spectrum “loops” around?

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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
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Actually it doesn’t loop around, but due to some clever hardware exploiting our visual sensory organs, it can be made to seem that way. In our eyes we have 3 types of sensors, reacting to (grossly) simplified red, green and blue. The bands of the sensors overlap slightly giving us the ability perceive ‘colors” of the spectrum as ratios of stimulation in each band. By having 3 light sources corresponding to the center frequency of each sensor, we can generate the perception of any color simply by varying the intensity of those three light sources. By modulating them with 3 sine waves 120 degrees out of phase, you can get the effect of a color wheel that perfectly loops around.

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.

That’s just your brain messing with you. When it sees red and blue light, but no green, the brain knows something is up because if it would be some color in the normal range between red and blue there would be some green in it. The absense of green tells the brain it has to be something else so it has to be purple.

A lot of how we perceive color depends on the brain and on what it expects to see.

Humans basically have 3 different types of receptors for light in different colors (with some overlap), usually called blue green and red (although that isn’t really accurate because each of them detects a range of colors and they have overlap) but based on how stimulated each of those receptors are, the colors will be interpreted. A strong signal on the receptors that react strongly to blue but a weak signal on the receptors for green and red makes it look blue, etc.

It’s not necessarily a coincidence. It’s directly implied by the fact that we perceive the full spectrum as white/colorless/neutral. Let me try and summarize the argument:

Basically, the brain interprets as “neutral” things that reflect the entire spectrum. Because our eyes has three types of receptors, this (simplified) physically translates to “all three receptors are receiving roughly balanced signals”. Given that this particular balance remains “neutral” no matter the total brightness of the signal, that leaves only two dimensions for color information. In other words, our perception of color will form *some* two-dimensional space centered on white.

I recommend taking a moment to look at [an actual graph of this two-dimensional space](https://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg), though the following argument does not assume any specifics about it.

I will gloss over the non-ELI5 topological details, basically what it boils down to is that this 2D spectrum of colors can essentially be further broken down into “colorfulness” and “hue” components, in a way that necessarily follows from the fact that “white” is in the middle. “Colorfulness” is basically “the distance from neutral”, i.e. the closer you get towards white, the more things will tend to fade or “lose their color”.

So if we have one axis for brightness (independent of color), one axis for “colorfulness” (distance from white), that only leaves one axis left for the actual *color* (i.e. what you describe as “the spectrum”). And because we already know that white is in the neutral / center position, this remaining axis must, in some sense, “loop around” white, such that you can make a full round trip through all colors without ever going through white.

**tl;dr** it would be mathematically impossible to have a “disconnect” in the color wheel without also destroying the property that a balanced spectrum is perceived as white/neutral.

p.s. in color science, what I called “colorfulness” is usually called either *saturation* or *chromaticity*, what I called “brightness” is often called *luminance* or *lightness*, and what I called “color” is usually called *hue*.

It doesn’t. This is why Magenta isn’t a real color. It’s a composite of wavelengths from opposing ends of the spectrum.

we get colorblind people like my friend at work who is red/green blind, they mix into each other. is their anyway to perfectly show someone sees colors differently but perfectly? so no color mixing they see red as blue but every ‘mix’ works as if it was correct?

its hard to explain. but is there a constant thats always 100% other than white or black to gauge it by?