When studying light, one of the most popular images of the visible spectrum is this:

Visible Spectrum of Light

Here is a classification of the range of the wavelenght that correspond to each color:

Range wavelenght of the colors from violet to red

Although it may be subjective wheter the boundaries between every color are well defined and actually it is a continuum, it is noticeable both in this table and in our perception that the wavelenght range, for example, of the colour "yellow" is very narrow (~570-590nm/[~20nm]), when compared to the big range of wavelenghts that are known by "red" (~620-750nm/[~130nm]).

To give another example, I'm gonna talk about a range of 100nm, starting from different points of the spectra. The blue we see at 480nm is perceived way differently from the colour we see at 580nm, that is yellow. Despite that, the red colour at 640nm seems pretty the same as the - also red - colour at 740nm.

What are the physical, biological and psychological reasons for this?

I found a website that explains that our eyes have more sensitive red and green cones than the blue ones and that is an possible explanation for why does yellow seems brighter. But I don't know if that explains also the reason why there are different ranges of what we perceive by the "same" colour. Here is what I found: https://rockcontent.com/blog/the-use-of-yellow-in-data-design/

Eyes cones

Sensibility of blue, green and red eye cones

Sensibility spectrum of the eye, overlapping blue, green and red cones


1 Answer 1


This is more of a biology question, especially since it deals with human perceptions like "red" "yellow" and "brighter."

Fundamentally, it makes no difference to add more "colors" beyond red if we can't distinguish between them. As we enter the red region, the short and medium cones aren't substantially stimulated by those long wavelength photons. There's no real good information to be gathered other than a single channel of "dim" to "bright." As such, there was no evolutionary advantage to having a perception of a color beyond "red." They all get lumped in.

As for why yellow appears "brighter," perceptually, that's a psychological thing. It has been found, over millions of years, that that's a range of wavelengths that is worth drawing our attention to. It's also in a region where our long and medium cones are far more sensitive to (which is one of those circular argument things. We've evolved cones that are good at sensing what is important to sense).

  • $\begingroup$ I agree that may be more biological, but maybe there are still some physical circunstances... For example, we may don't differenciate red colours because there are some sort of materials (food, blood, etc.) that reflects 620nm-750nm and it is not as important to differentiate it as it is to differentiate blue from yellow. Maybe what I wanted to know is the kind of material that may be important and how its interaction with light, (the reflexion of a wavelenght) influences. It may be something simple, like "we don't differentiate red because food has iron and it reflects from 600 to 700nm". $\endgroup$ Commented Apr 29, 2021 at 16:28
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    $\begingroup$ @GabrielHenriqueArmandoJorge Biology.SE may be a good place for that. You're more likely to find people with that knowledge there. For example, they may understand why we are trichromats while some birds are tetrachromats. $\endgroup$
    – Cort Ammon
    Commented Apr 30, 2021 at 5:47

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