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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?

Qmechanic
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Dmytro
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  • 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. –  Aug 05 '18 at 11:45
  • Possible duplicate of How does light combine to make new colours? – knzhou Dec 16 '18 at 15:57
  • @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? – Dmytro Dec 17 '18 at 05:53
  • @Dmitry All of those questions are answered in the linked question! – knzhou Dec 17 '18 at 11:47
  • @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. – Dmytro Dec 17 '18 at 19:53

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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.

Lichen
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  • so you are saying it might be possible that we can see yellow but there is no yellow frequency as defined by the chart? – Dmytro Aug 05 '18 at 11:42
  • Colored compounds of many transition metal complexes work in a similar manner, if I'm not mistaken. –  Aug 05 '18 at 11:48
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    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. – S. McGrew Aug 05 '18 at 14:32
  • @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. – Dmytro Aug 05 '18 at 14:44
  • @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. – MaxW Aug 05 '18 at 16:11
  • yes but is the emission of green and blue equivelent to yellow light or are they different but we perceive them similarly and if different, do they overlap – Dmytro Aug 05 '18 at 16:13
  • It *doesn't* activate the cones in the eye in quite the same way as yellow would ... the yellow you get by mixing green and red light is not quite as pure as the yellow from a single yellow wavelength. – Peter Shor Aug 05 '18 at 19:04
  • What happens when red and green light hits a surface and bounces off that surface into your eye, rather than directly into your eye; Does it become true yellow? If yellow and red green were different, shouldn't there be materials that respond differently to red and green hitting them and yellow hitting them? – Dmytro Aug 21 '18 at 03:27
  • or rather, shouldn't there be yellows that, when reflected off a surface, do not mix with cyanish pigment to produce green(indicating that there is no red in the green), or do not mix with magentaish pigments to produce red(indicating that there is no green)? – Dmytro Aug 21 '18 at 03:38
  • They overlap quite copiously because we are only seeing just less than 1 octave of light,from 400 to 700 nm, whereas we can hear at least 7 octave of sound (100, 200, 400, 800, 1600, 3200, 6400, 12800hz). – Tomachi Jan 19 '23 at 00:11
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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.

niels nielsen
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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.

anna v
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  • 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)? – Dmytro Aug 05 '18 at 18:44
  • 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 . – anna v Aug 06 '18 at 03:54