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I know that if I look at an electromagnetic wave with a wavelength between 400 and 700 nm, I will see it as having some color. Is the converse true? I.e., can every color be generated by a single electromagnetic wave (with a given wavelength)? Take, for instance, the RGB space. Every color there is generated by means of three waves (one red, one green, one blue). Could each of these colors also be generated by a single wave?

As an example, consider the visible light spectrum illustrated in the following figure. Adding a wave with a wavelength of approx. 650nm (red) and one with a wavelength of approx. 470nm (blue), I get the color magenta. From the picture, it would seem that I could also get this color with a single wave with a wavelength of approx. 420nm. So, could I do this for every conceivable color?

enter image description here

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  • $\begingroup$ How about "white"? $\endgroup$
    – BowlOfRed
    Commented Apr 20, 2018 at 16:25
  • $\begingroup$ Or how about brown? $\endgroup$
    – user137289
    Commented Apr 20, 2018 at 20:00
  • $\begingroup$ White is not a color, as I understand it. And what about brown? What's special about it? $\endgroup$
    – LGenzelis
    Commented Apr 20, 2018 at 20:27
  • $\begingroup$ @LGenzelis Brown, among other colors, is composite. $\endgroup$
    – Kyle Kanos
    Commented Apr 20, 2018 at 23:25
  • $\begingroup$ @KyleKanos but so is magenta, as in the example I gave in the post (after editing it). $\endgroup$
    – LGenzelis
    Commented Apr 21, 2018 at 17:00

3 Answers 3

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The short answer is no. While advertisers like to use the phrase "every color in the rainbow" (which is what the single wavelength colors are) to suggest every color imaginable, this is in fact not the case. There are a plenty of colors that are not in the rainbow - there's no brown or pink, for example, or magenta or cyan. All of these colors need the output intensities for our three types of cone cells to in a balance that is not achievable with a single wavelength.

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No, because your eyes are sensitive to three different segments of the spectrum.

Here's a CIE 1931 color space from Wikipedia. This is an estimate of all the hues that human with human-normal color vision can see. The curved line that bounds left and upper-right border of the space are colors that can be generated by monochromatic light -- everything else must be generated by at least two different wavelengths.

This is a 2D chart, because it's leaving out intensity -- because a typical human has three different types of light sensor active during daytime, we should be able to see three dimensions of color -- this is why TV and computer screens can get by with just three colors of emitter.

  • Most mammals, and some humans, don't have a "green" receptor -- just red and blue. So for them, monochromatic light would replicate all the colors they can see.
  • Most vertebrates (i.e., nearly all non-mammalian vertebrates) have four distinct color receptors, and their "green" receptor isn't our* green receptor. So to them, our TV screens would be lacking in a full gamut of colors, and what's there would be weird.
  • Something that's left out of all the "humans with aliens" science fiction that I've ever read is the whole color vision thing. Somehow, I don't think it's a slam-dunk that Vulcans, Andorrians, Cylons, Hut, etc., etc., will be able to just read the same view screens as we do -- and visa versa.

CIE 1931 Color Space

* I'm assuming this is only being read by humans.

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So I think what you're asking is if a EM wave packet that includes multiple wavelengths can be perceived as the same as a single coherent EM wave. If that's the case, I think this problem has more to do with our biology and what our eyes see and our brains interpret than pure physics. From a purely physical perspective, a single coherent EM wave is qualitatively different than a wave packet that contains many different EM waves. One could always conduct physical experiments to distinguish between the two (e.g. run the wave through a spectrometer, or a diffraction grid). Whether our eyes can distinguish between the two I'm not certain since I'm a physicist, not a biologist.

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  • $\begingroup$ "So I think what you're asking is if a EM wave packet that includes multiple wavelengths can be perceived as the same as a single coherent EM wave". I edited my question, so as to make it more clear. $\endgroup$
    – LGenzelis
    Commented Apr 20, 2018 at 20:38
  • $\begingroup$ I'm still getting the same interpretation of what your question is. Was there something else you wanted to know? "Every conceivable color" is also quite ill defined...I feel like your question is more of perception than physics. From a physics perspective, my answer doesn't change. A single coherent wave is qualitatively different than two waves combined in a wave packet. $\endgroup$
    – enumaris
    Commented Apr 20, 2018 at 20:41
  • $\begingroup$ But I am not interested in every quality of the waves, just their color. It is as if I asked if I can get an amount of sand that weights the same as some amount of iron, and you answered that the question is not about physics because sand and iron are qualitatively different. $\endgroup$
    – LGenzelis
    Commented Apr 21, 2018 at 16:54
  • $\begingroup$ But color is a perception issue isn't it? A biological perception of what color is. I addressed that in my answer as well...maybe I'm misunderstanding...but what do you mean by a purely physics definition of color? $\endgroup$
    – enumaris
    Commented Apr 21, 2018 at 17:30
  • $\begingroup$ To be honest, I am not sure. I am not a physicist. I am just physics-curious. If this question can not be answered here, it's probably pointless to keep trying. I will save the question until I run into a neurologist (or maybe psychologist?) who has a background in physics. $\endgroup$
    – LGenzelis
    Commented Apr 22, 2018 at 20:36

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