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I am trying to understand how color works, and I am curious whether the yellow we perceive when our eyes are hit by red and green light at the same time is the same yellow that is at the exact yellow frequency.

What I mean is that I read recently that white does not have a frequency by itself, but rather that we perceive white when we perceive all frequencies of light at once(at least ones we are sensitive to).

What confuses me is that based on Wikipedia chart, colors of visible liht spectrum

red has a frequency of ~480, and green has a frequency of ~550, whereas yellow has a frequency of ~540. So i am not sure whether they add up in some way to become equivelent to yellow, or if the presence of red and green makes our eyes perceive yellow without there being any yellow.

To clarify, I mean that yellow has its own frequency and wavelength, and red and green have their own frequencies and wavelengths, so when we see yellow, is it equivalent to seeing red and green, or can we perceive red and green but if we capture the frequency of yellow of the light, there may not necessarily be any yellow there. Also, if they are different yellows, then what happens if red, green, and yellow is present at the same time?

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  • $\begingroup$ Can you edit the title a bit? I'm kind of confused by it... The first paragraph ("I am curious whether the yellow we perceive...") is a much clearer question. $\endgroup$
    – user191954
    Aug 5, 2018 at 11:45
  • $\begingroup$ Possible duplicate of How does light combine to make new colours? $\endgroup$
    – knzhou
    Dec 16, 2018 at 15:57
  • $\begingroup$ @knzhou i don't care how light combines to make new colors, I want to know (1) whether yellow/yellow frequency is the same as yellow/red and green frequencies without yellow, (2) whether it is possible to have a material that looks black under both green and red light, but still looks yellow under pure yellow light, (3) what happens if yellow light in yellow frequency is mixed with green and red frequencies, will it get brighter, will it look the same? $\endgroup$
    – Dmytro
    Dec 17, 2018 at 5:53
  • $\begingroup$ @Dmitry All of those questions are answered in the linked question! $\endgroup$
    – knzhou
    Dec 17, 2018 at 11:47
  • $\begingroup$ @knzhou I am struggling to find the answer to the last ones, what happens when yellow is mixed with red and green(is it going to be brighter? no difference? compared to pure yellow or pure red and green light), and I am not sure whether there can be a material that only reflects yellow wavelength but absorbs red and green wavelengths. $\endgroup$
    – Dmytro
    Dec 17, 2018 at 19:53

3 Answers 3

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Great question. This page (https://en.wikipedia.org/wiki/Color_vision) indicates that there are three cones in the human eye which absorb and hence are sensitive to different areas of the spectrum. Our metal perception of colour is then a ratio of what is being received by these cones. By mixing red and green, yellow is perceived not because a yellow frequency is produced by mixing or beating, but because if you get the ratio right it activates the cones in the eye in the same way as yellow would. This isn't a full answer because I don't understand it fully. It looks like it's more a biology question than a physics one - hopefully digging into colour vision will answer it fully.

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  • $\begingroup$ so you are saying it might be possible that we can see yellow but there is no yellow frequency as defined by the chart? $\endgroup$
    – Dmytro
    Aug 5, 2018 at 11:42
  • $\begingroup$ Colored compounds of many transition metal complexes work in a similar manner, if I'm not mistaken. $\endgroup$
    – user191954
    Aug 5, 2018 at 11:48
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    $\begingroup$ That's correct: The brain interprets "this light activates both red and green sensors" as "yellow". A pure light frequency in the 510-540 THz range will activate both red and green sensors in the eye and produce a signal that is almost identical to the signal produced by a mix of light at, say, 530 and 560 THz. This is because the sensors in the eye respond to overlapping ranges of frequencies. $\endgroup$
    – S. McGrew
    Aug 5, 2018 at 14:32
  • $\begingroup$ @S.McGrew But would overlapping red/green with yellow be any different than only red/green or yellow? i would imagine it would seem to be a brighter yellow, but at the same time I'm not sure it works the same way waves resonate since it's different frequencies. $\endgroup$
    – Dmytro
    Aug 5, 2018 at 14:44
  • $\begingroup$ @Dmitry - Evvere used a color computer monitor? The monitor can only make red, green, and blue colors. Other colors that our mind perceive are due to mixing of the three colors. $\endgroup$
    – MaxW
    Aug 5, 2018 at 16:11
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Because the human eye has sensors that detect only three color bands as pointed out by S. McGrew and MaxW, it is indeed the case that your brain, retina, and optic nerve are wired to tell you that you are seeing "yellow" when there are no photons at all of that energy entering your eye. This effect is called metamerism and is extensively dealt with in the field of physiologic optics.

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As the other answers state, color perception and frequency of light are two different things. In this plot

color perception

The properties of color which are inherently distinguishable by the human eye are hue, saturation, and brightness. While we know that the spectral colors can be one-to-one correlated with light wavelength, the perception of light with multiple wavelengths is more complicated. It is found that many different combinations of light wavelengths can produce the same perception of color. This can be put in perspective with the CIE chromaticity diagram.

So it is not a simple one to one correspondence.

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  • $\begingroup$ what happens when multiple combinations trigger at once, would it yield brighter yellow? also based on what you said it's many to one correspondence, right (many ways to make us think we see yellow)? $\endgroup$
    – Dmytro
    Aug 5, 2018 at 18:44
  • $\begingroup$ by definition and eye construction, we see yellow. It so happens that the spectrum of pure frequencies coincides with what our brain has labeled "yellow". If you are really interested you should read up on the links in the link given, which explain how the plot is made, and then you could see what would come up with more than two frequencies adding up , which should be white for adding all of them . $\endgroup$
    – anna v
    Aug 6, 2018 at 3:54

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