Let's talk about the visible light "rainbow"

In almost all depictions of the electromagnetic spectrum, visible light is shown as a rainbow, and adjacent parts of the spectrum are monotone in color - there is no UV or infrared rainbow to be seen.

enter image description here

This is justified - the vast majority of people (animals are a different story) are confined to visible light, so we cannot simply "explain" what an unseeable color looks like. Therefore, it's easiest to assume every wave longer than visible light is representable by "red" and shorter by "violet". We can't color with unseeable colors, so we don't make more rainbows.

In the context of fictional creatures evolving to see different parts of the spectrum (let's ignore the challenges with that for now), can we identify groups of fairly similar waves - the "rainbows" for UV, infrared, etc. to determine the "colors" these creatures might see - or is there really just monotoneness?

Not a duplicate of Colors of Things Outside the Spectrum which asks what humans would see if light were absorbed in a specific way, instead of what other organisms would see if absorption remained generally the same as it does now.

  • $\begingroup$ We can "see" these wavelengths, just not directly. Look at the dial of an old fashioned radio, for example. And while the image is translated for our use, the view screen of a thermal imager shows different "colours" of the infrared spectrum. $\endgroup$
    – Thucydides
    Dec 10 '16 at 4:57
  • $\begingroup$ @Thucydides Radios are too picky - they don't establish "blue", rather "color #1" "color #2" "color #3" with no definition of groups that look the same optically. Thermal imagers are tuned, and vary between builds on the wavelengths chosen - they do not rely on "thermal colors" based on waves, rather on making the decision of how to group them (to the best of my knowledge) $\endgroup$
    – Zxyrra
    Dec 10 '16 at 5:04
  • $\begingroup$ U mean false color? $\endgroup$
    – user6760
    Dec 10 '16 at 10:46
  • $\begingroup$ There might be enlightenment to be had talking to AmateurRaido hams. They qualitatively characterize the different frequencies set aside in the FCC bandplan. I would try to find an Elmer who is blind and start a conversation. $\endgroup$
    – user25818
    Dec 11 '16 at 1:26
  • $\begingroup$ Note that there are non-spectral colours, i.e. colours not in the rainbow, such as purple and magenta. So, the relationship between what we see and light is more complex than just wavelength. So for fictional creatures you can imagine it similarly complex. :-) $\endgroup$
    – Pablo H
    Jul 21 at 21:28

There are no "similar waves" we just see the three base colors (RGB) becasue we have three types of color receptors. We have receptors for three, many other mammals have two base color, birds and reptiles see four base colors, some arthropods have a dozen. the number of different types of photoreceptors in the eye determine the number of colors you see, not anything about the light. organisms can have more base colors and will literally see more colors in the same stretch of the spectrum we see. The opposite is true as well red and orange are the same color to dogs just as there are several colors inside green to things that have multiple different types of "green" photoreceptors.

Colors are not created by light they are created by how our eyes sense light. Distinct colors will exist everywhere on the spectrum for eyes that can see them. The rainbow never stops just our ability to see it. There are more diverse rainbows inside out rainbow that we can not see. We are all almost completely colorblind, even with the narrow visible spectrum we can't see most of the colors.

  • $\begingroup$ I know some achromats who actually aren't completely color blind but can pretty much just register either purple, or a single color for everything else. It makes shopping for gifts much easier. Now, what a tetrachromat or pentachromat might call the colors would be an interesting fictional exercise. Probably best to come up with one word for the new primary color (maybe "ultra" if ultraviolet or "infra" if "infrared" and then to use binary names like blue-ultra rather than coming up with distinct names for new primary color combinations. $\endgroup$
    – ohwilleke
    Dec 10 '16 at 7:57


Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cells in the eye. Most people have three cones (making them trichromats), which enables them to see about one million colors. But tertachromats have four cones, so their eyes are capable of picking up dimensions and nuances of color-an estimated 100 million of them—that the average person cannot.

Humans are usually trichromats, but recent studies suggested that 2–3% of the world's women might have the type of fourth cone whose sensitivity peak is between the standard red and green cones, giving, theoretically, a significant increase in color differentiation. Another study suggests that as many as 50% of women and 8% of men may have four photopigments and corresponding increased chromatic discrimination compared to trichromats.

Concetta Antico

Concetta Antico is a tetrachromat and so is able to see 100 times more colors than the average person:

When Concetta Antico looks at a leaf, she sees much more than just green. “Around the edge I’ll see orange or red or purple in the shadow; you might see dark green but I’ll see violet, turquoise, blue,” she said. “It’s like a mosaic of color. It’s shocking to me how little color people are seeing,” she said.

Same Neurology as Trichromats

Interestingly, although tetrachromats have more receptors in their eyes, their brains are wired the same way as a person with normal vision. So how can a brain like Antico’s change to see more colors? Like anything else, practice makes perfect, even when it comes to neural pathways.

Researchers Jameson and Winkler are on the hunt for more tetrachromats in order to better understand how their brains work. Jameson became fascinated with how people are able to form and communicate concepts, especially when the way they perceive the world can vary so widely. “If you have an extra cone class in the retina, that greatly complicates how that signal might be taking shape as it leaves the retina. We want to understand how that’s happening,” she said. This likely has to do with how the brain wires itself when it receives certain signals frequently over time—a concept called neuroplasticity. Lots of studies about neuroplasticity in animals and some in humans have shown that two individuals with the same capacity for visual perception can have drastically different vision later in life just based on what they were exposed to early on. Researchers still aren’t totally sure why this is the case. “One possibility is that the system learns how to use these signals—the wiring creates the proper code so they can be used in the cortex,” Jameson said.

So even though many more tetrachromats may exist in the world, they may not have exceptional color perception, because they haven’t trained their brains to pay attention. Antico, in this case, presents a rare exception. “I was different than a regular 5-year-old — I was painting at age 7, I was so fascinated with color,” she said. For years, she was exposed to exceptional color, so her brain became wired to take advantage of her tetrachromacy.


Assigning color names to certain wavelengths really is a completely arbitrary process and can be quite different in different cultures around the world. A very common example are languages that don't distinguish between green and blue, but there are many other variants.

The seven rainbow colors are just as arbitrary and you could very well say that it has only five colors: Red, organge, yellow, green, and blue. Whether you want to call light blue "indigo" and deep blue "violet" (there is no purple in the light spectrum) has nothing to do with specific wave length.

Creatures with eyes that are sensitve to different wavelength than humans would be able to distinguish between them just as well as humans do with red, green, and blue, and most likely also have the ability to see combinations of different wavelength (like we see orange).

The image in the question is actually wrong: The color spectrum it shows has all the infrared frequencies in red and the ultraviolet frequencies in shades of purple. In reality these would be just black to human eyes.


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