I was watching a multi-colored loading icon, and it piqued my interest.

Red, orange, yellow, green, blue, indigo, violet.
(Is indigo a real color? I don't know, but that's what I was told growing up)

Is there a specific reason for the wavelengths associated with these colors? Yellow is a color, and green is a color, and what's in between seems to be yellow-green or whatever name you want to call it. But why isn't that color that lies between yellow and green what we choose to identify? Or for any pair of adjoined colors--why are red and orange distinct colors, instead of red-orange and orange-yellow?

Is it arbitrary that we chose these spots on the spectrum to identify as red, orange, yellow, and green? Or is there a scientific reason we chose these wavelengths?

I have a basic understanding of the electromagnetic spectrum and visible light, but nothing more. My apologies if I made incorrect assumptions or did not word things correctly.


closed as unclear what you're asking by Carl Witthoft, John Rennie, Kyle Kanos, Norbert Schuch, ACuriousMind Feb 26 '16 at 15:26

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    $\begingroup$ The colors are arbitrarily assigned. This question is more about visual perception and not about physics. $\endgroup$ – Floris Feb 25 '16 at 19:47
  • $\begingroup$ @Floris I mean, somewhat arbitrary, but not completely, right? I'd also argue that visual perception is part of physics, as it's the "receiving" side, and thus reflects how reality is experienced. I think the question has physical relevance. $\endgroup$ – YungHummmma Feb 25 '16 at 19:56
  • $\begingroup$ Indigo is definitely the most arbitrary -- it's included in order to have a vowel between B and V for acronym purposes. $\endgroup$ – user10851 Feb 25 '16 at 19:58
  • $\begingroup$ And even violet is not in the rainbow, it actually ends with blue. $\endgroup$ – rodrigo Feb 25 '16 at 20:23
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    $\begingroup$ I'm voting to close this question as off-topic because it isn't about physics $\endgroup$ – John Rennie Feb 26 '16 at 9:58

There's nothing physical about the different visible wavelengths. Like you correctly stated, they're all fundamentally the same thing.

However, you could make the argument that there are some biological differences. The reason for that is that, when you see a color, it's not just a result of the wavelengths that are making it to your eye -- it's also how your eye responds to different wavelengths!

So take this diagram for example:

enter image description here

That shows how your different cones respond to different wavelengths.

So one answer to your question might be that, the wavelengths we choose to call the colors we do, are a result of those responses. For example, maybe we have much better resolution between colors in the blue than in the red (or vice versa, I don't know), i.e., two wavelengths 20nm apart look very different in one range, and pretty much the same in the other.

  • $\begingroup$ Wittgenstein had much to say on the question of color and whether it was the same to different people. But that is philosophy, not physics. $\endgroup$ – Jon Custer Feb 25 '16 at 20:07
  • $\begingroup$ @JonCuster Sure, but I'm talking about even just with respect to one person -- I'm saying that if you wanted to choose a set of, say, 7 points in the visible spectrum to define as colors, there's probably an objective way to choose them, based on the (average) human eye's responsivity in the different regions, so that those points probably aren't equally spaced. And responsivity is a pretty physical question, especially with regards to detectors. $\endgroup$ – YungHummmma Feb 25 '16 at 20:28
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    $\begingroup$ By now I just think in terms of standard laser wavelengths. Odd, perhaps, but effective... $\endgroup$ – Jon Custer Feb 25 '16 at 21:09

When Isaac Newton did his work on the phenomenon of colors produced by a prism, he was inspired to copy from the field of music, and denoted "seven colors in an octave"; the ear can actually hear several octaves, but the eye only sees one.

This somewhat forced analogy is what leads to the seven colors, ROYGBIV.

For the artistically inclined, visit http://www.webexhibits.org/colorart/bh.html.


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