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I suddenly thought of a interesting thing:

say there are two light rays with the exactly the same colour, for example, purple. Red light ray and blue light ray add up to purple. Also, there is a monochromatic light of purple. Since they are the same colour, they must have the same frequency. But of the two light rays, one is the addition of two different frequencies (red and blue), the other is monochromatic (i.e. the frequency of purple colour).

The question is: what's the mechanism of addition of different frequencies of lights? Does my idea mean that, for one colour, there are two types of it? One is made from addition, and the other is monochromatic?

Of course it likely that my idea is wrong. I am willing to learn the truth of this interesting phenomenon. Could anyone help me out?

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    $\begingroup$ Where did you get the idea that there is a purple monochromatic color? $\endgroup$
    – Ed V
    Oct 21, 2021 at 11:44
  • $\begingroup$ There are lots of light color questions and answers here, like this one: physics.stackexchange.com/q/588617/313612. $\endgroup$
    – Ed V
    Oct 21, 2021 at 11:50
  • $\begingroup$ Thank you very much! The idea just occurred to me suddenly :) $\endgroup$
    – bruce mao
    Oct 23, 2021 at 12:39

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This is a perception problem. Though there are infinite wavelengths in the visible region, our eyes have cells called cones that are sensitive to only three colours: red, green and blue (see sensitivity graph below)

enter image description here

So each colour is associated with three values of excitation $(R,G,B)$. For example the wavelength $500$nm is associated with $(0.25, 0.3, 0.1)$. The wavelength $650$nm is associated with $(0.2, 0.02, 0)$ and so on. Our brain senses these three values and back calculates the wavelength and that’s what we perceive.

Though you may trick the brain into thinking there’s one one colour by mixing appropriate amounts of red, green and blue (this is what most computer screens, printers do), the light is physically different. The difference can be seen by letting the light pass through a spectrometer.

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  • $\begingroup$ isn't there a small bump of the S sensor at higher wavelengths also, causing the sensation of purple? $\endgroup$ Oct 21, 2021 at 13:04
  • $\begingroup$ Thank you very much! $\endgroup$
    – bruce mao
    Oct 23, 2021 at 12:39

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