How can RGB reproduce all (most) colours?


While learning about photography I've realised that there's something I can't explain with the physics I was taught at school.

What I do know is that pure red, green and blue have specific frequencies. And that combining the colours produces white. And although I don't know exactly why, I've read that red, green and blue together can approximate most colours - it has a limited "gamut" although personally I can't spot the deficiency.

My main questions are...
1) How can those frequencies produce colours at frequencies different to themselves? e.g. yellow
2) And I thought white was the visual equivalent of white noise (broad band) so how can that be produced with RGB?

Has it got anything to do with how the receptors in our eyes work? Back to biology now lol I remember there existing RGB sensitive cells... it's sounding less and less like a coincidence! Does that mean a green receptor would react to frequencies on either side? Just guessing now...
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Homework Helper
Your 3 is right, it has everything to do with receptors. IOW it is a question of physiology not physics.

Just rough principle since I think the explanation is here on this site somewhere in more detail and very many textbooks, you have 3 types of receptors with response spectra spread but centred on R G or B. Some pure wavelength you ask about will produce a given response on all 3, proportions different according to that wavelength. So each pure wavelength corresponds to a unique combination for you neural biocomputer amd it distinguishes thousands of them.
Sorry for the physiology post :p

Thanks - now I know what to look into next :)


Homework Helper
Wiki has a reasonable article. The 3 sets of receptors on most human eyes, each with maximum sensitivity at some frequency, but the sensitivity peaks are not "exactly" red, blue, and green.
it has a limited "gamut" although personally I can't spot the deficiency.
Compare the "orange" patch in wikipedia's article (The colors of the visible light spectrum), which is (R=255, V=128, B=0) and has a 100% saturation, to the color of an orange skin or a clementine skin !

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