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enter image description here

We have 3 color receptors in our eye, so assuming the above picture is precise we are checking for 3 wavelengths: 450nm, 540nm and 700nm.

Pink for example has an RGB value of

  • R (700nm): 1
  • G (540nm): 0
  • B (450nm): 1

Violet has an RGB value of

  • R (700nm): 0.5
  • G (540nm): 0
  • B (450nm): 1

However Violet has a wavelength of 400nm which is lower than the Blue receptor. So how can it activate the red receptor when red is clearly not active at that frequency?

Does that mean Ultraviolet appears blue to the human eye? Why is it constantly painted as violet in all frequency graphs?

enter image description here

The question was first duscissed here (https://physics.stackexchange.com/questions/486560/what-frequency-is-pink-color) but did not receive the answer I need.

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  • $\begingroup$ related question. biology.stackexchange.com/questions/51870/… $\endgroup$
    – John
    Jun 22, 2019 at 13:31
  • $\begingroup$ Ever see a rainbow, or sunlight that's passed through a prism? The high-frequency end of the visible spectrum is painted as violet because that's what it looks like. $\endgroup$
    – jamesqf
    Jun 22, 2019 at 16:01

1 Answer 1

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this is a chart of the light activation of the light sensitive cells in the human eye.

enter image description here

see that little blip up on the tail end of the red cone, that minor ranges means the extreme blue end of the spectrum activates blue cones but also has a chance to activate red cones. That is why you can sometimes see magenta tinges in bluest of blue light. There is also a sensitive issue as blue cones are the least sensitive of our cones so it requires a stronger blue light source to activate it than the equivalent light on other cones, since red is more sensitive a strong blue and a weak red source will blend to about equal as far as our eyes are concerned so at the bluest end of the spectrum, where the blue activation is at its weakest it does not take much to activate the blue and red "blip" equally meaning all sources at that extreme end blend into magenta.

Also magenta itself is not actually part of the spectrum, it can be produced in two ways, the first I discussed above, hitting the far end of that little blip perfectly. The second and most common requires light from two disparate and different parts of the spectrum in other words not a single wavelength. It is blue light and red light without green light, it is not a real color but an artifact of how our eyes see light. Your cones can't tell how they are activated just that they are. Yellow and cyan (and every other common human color)on the other hand can be produced by a single wavelength, which may or may not activate multiple types of cones. You can find more information here

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    $\begingroup$ Really impressed by the straightforwardness of the linked article. I quote Einstein here "If you can't explain it simply, you don't know it well enough" Thanks for that!! Fascinating!!! $\endgroup$
    – AzulShiva
    Jun 22, 2019 at 13:55
  • $\begingroup$ This is incorrect. There's no bump in the sensitivity of the L cones. This misconception arose from a bump in the X function of the CIEXYZ colour space. But X, Y and Z were never meant to correspond to cone sensitivities. $\endgroup$
    – Oscar Cunningham
    Jul 13, 2020 at 5:38
  • $\begingroup$ @OscarCunningham I see... can you edit the answer or write a new one? I am in no position to do so. $\endgroup$
    – AzulShiva
    Apr 5, 2021 at 20:05
  • $\begingroup$ So what is the answer to this then? I have never seen that activation spectrum from any other source. If there isn't that hump in the red cone's spectrum, then it seems the OP's question remains unanswered. $\endgroup$
    – Joel Croteau
    Jun 2, 2022 at 17:52

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