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Why do we see violet beyond blue in the spectrum?

  1. Jun 24, 2011 #1

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    Our eyes have 3 types of cones to perceive color, one for red, one for green, and one for blue.

    On the one end of the spectrum we see red.
    But on the other end of the spectrum we do not see blue, but beyond blue we have violet.
    How is this possible? :confused:

    A bit of research found these images and their related wiki articles.



    They appear to contradict each other!
    What is going on? :confused:

    Btw, this is a spin-off of the thread: https://www.physicsforums.com/showthread.php?t=508845 in Classical Physics
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  3. Jun 24, 2011 #2
    I don't understand what the problem is with that. 'Violet' is simply the name we ascribe to the color we see beyond (higher frequency than) blue. While the central sensitivity of the blue-cone is a little lower in frequency, we are still able to see a little higher.

    If you read the articles, you'd see that those two plots are describing quite different things. First: they both particular models (one of them is from the 30's---from a particular study).
    Second: One describes the overall human response in a particular study, while the other is the cone's response in particular (not to mention this is a normalized plot).

    Btw, this is a spin-off of the thread: https://www.physicsforums.com/showthread.php?t=508845 in Classical Physics[/QUOTE]
  4. Jun 24, 2011 #3

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    Are you saying that the blue-cone is actually a violet-cone?
    And that what we see as blue is actually a mix of violet and green/red?

    Ermm, but the 30's CIE model at least explains violet beyond blue in a sense, while the cone-response-curves does not?!
    Last edited: Jun 25, 2011
  5. Jun 25, 2011 #4


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    the visible spectrum is ~390-750 nm, violet light is 408 nm.

    you see a low response from the cone sensitivity plots and you assumed that it meant we couldn't detect it, but there's a lot of processing that goes on between your cones and your subjective experience of violet.

    three points:

    Exactly how color gets transcribed so crisply is still largely a mystery (according to my Purves Neuroscience text) but the first point of my post is to say that the receptor response may just be a coding scheme to be further processed and not reflect the way colors are percieved subjectiviely.

    The second point. Maybe light in natural conditions is dominated by violet wavelength light, so our receptors respond less to normalize the intensity with respect to the detection of other colors.

    Finally, remember that you can get purple by mixing red and blue in paints, much like you see grey when you pattern black and white with small enough bits. This doesn't require your receptor to detect 408nm light.
  6. Jun 25, 2011 #5

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    Err, no, I didn't assume we can't detect 408 nm. I know we can, because we can see the violet light.
    It's just that I thought that only the receptor for blue would be triggered, so it should appear blue. Why doesn't it?
    Why does violet have an apparent tinge of red in it, when red is on the other side of the visible spectrum?
  7. Jun 25, 2011 #6


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    ah, my apologies.

    That's because the three receptors are poorly named "red" "blue" and "green". As you can see from the plots, each receptor actually can detect on a large range of frequencies. But any frequency location (looking at a vertical slice on your plot) has a unique combination of inputs from the three types of receptors, which can (theoretically) be decoded by a downstream neuron as a particular color state.
  8. Jun 25, 2011 #7

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    Are you saying our neurons are cross-wired?
    Or that our brain is making things appear different than they are?

    Actually, that would seem rather odd to me, especially since it appears to be common among all humankind.
  9. Jun 25, 2011 #8
    No. The peak of the sensitivity centroid is at blue, there is still some sensitivity at higher frequencies--at violet.
  10. Jun 25, 2011 #9

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    Yes of course, the peak is at blue. :)

    But suppose only the blue cones are triggered, would that fool our brain into seeing violet?

    In other words, if we would disconnect the red and green cones (if we could), would we then perceive blue light as violet?
  11. Jun 25, 2011 #10


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    Only the blue cones triggered isn't enough information.

    We also must discuss magnitude, which varies with frequency (the "blue" cone still varies it's response across the range of frequencies it responds to, giving downstream neurons information to decode.
  12. Jun 25, 2011 #11
    I think triggering blue cones gives rise to violet and red cone stimulation and bue cone stimulation is required for blue, the names of the cones do not match up with the actual colour percieved, the names arise from the originators of the theory.
    This might be of some use:
    http://books.google.co.uk/books?id=...epage&q=violet blue distinguish cones&f=false

    The text is also useful, not just the diagram.
    Sorry if it's not!
  13. Jun 25, 2011 #12
    Something to note. I work with 350 nm lasers. You can see them dimly. Also, in the tropics, where there is a lot of UV at mid-day people take lots of pictures to show the folks at home. They never come out as brilliant as expected. The eye is taking in and sensing a lot of the UV but the LCD monitor of your screen just does not reproduce it. So my vote is for more perception of the UV than we give ourselves credit for.
  14. Jun 25, 2011 #13


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    The other cones still pick up light, even though it isn't the peak absorbancy. How well these cones are absorbing the light from a particular area of the visual field is how the down stream processing happens.

    Different parts of your visual field (for color, movement, shape, etc) are coded and processed in by specific ensembles of neurons in your occipital lobe.

    Let my try a simplified example to see if that helps.

    Suppose you were looking at a purple grape in the upper left quadrant of your visual field. Those rods and cones on your retina, taking in that specific little area of light are mapped further down stream in the brain. So that specific retinal ganglion cell (and the rods and cones its processing for) gets activated.

    All the cones get activated, whether they be blue, red or green. The "blue" cones however absorb the light best, while the red and green absorb it not as well. This "comparison" between absorbancy is the information "sent on" by that part of the receptive field.

    Color processing starts there at the retina and gets more refined as it is "passed back" along the visual tract (optic nerve, chiasma, radiations, lateral geniculate nucleus and eventually visual cortex). The visual cortex has different layers and complex processing happens in a couple of them IIRC.

    In the primary visual cortex, some ensembles of neurons respond better to information derived from ganglion cells from from specific wavelengths of light, better than others. This differential excitation of neurons in the visual cortex gets further passed along and further processed. I don't think anyone has worked out the exact details as of yet.

    Eventually this information gets passed on to secondary and multimodal processing areas of the brain--Which integrates many kinds of sensory information (say for instance, the color of flame, the shape of flames, the sound and smell of something burning, etc). Which allows you to have your conscious perception of what your senses are taking in (though the brain does a great deal of filters your senses without your consent).
  15. Jun 26, 2011 #14

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    Thanks for all your answers. I'm still taking them in.

    One important point I got from the other thread is that we hardly ever see true violet in practice. Basically only in a rainbow.

    And to get violet in a picture, you need a chemical that has exactly the spectral color violet, which is not readily available.
    So in a photograph you always see only a subset of the visible colors.
  16. Jul 1, 2011 #15
    Hmmmmm........The compact flourescent bulbs that will soon be mandated sure do have a lot of output in narrow bands.

    It seems they radiate a lot more energy that is not usable by our eyes than the former incandescent bulbs.

    check out the graphs

    http://www.gelighting.com/na/business_lighting/education_resources/learn_about_light/distribution_curves.htm [Broken]
    Last edited by a moderator: May 5, 2017
  17. Nov 24, 2012 #16
    The eye’s red receptor has a secondary absorbtion/response at short wavelengths in the farther indigo overlapping the blue receptor. The brain interprets naturally this as the mix of red and blue i.e. violet in this case, and so it looks like purple to the brain.

    This is not easy to find on the web.

    Most pages about color are mistaken in this regard (as are essentially all school teachers and people that believe that the color circle is a physical reality – it is a brain reality!). I use the “question”? “Why does violet look like purple” in my university course on “innovation and creativity” as an example of the “unasked question” that leads to scientific progress when it is finally asked.

    see: http://photo.net/learn/optics/edscott/vis00010 [Broken]
    Last edited by a moderator: May 6, 2017
  18. Nov 25, 2012 #17

    jim mcnamara

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    Mantis shrimp have 12 primary vision pigments with 4 additional "filtering" pigments:
    Note several citations in the biblio. Researchers believe that these shrimp have excellent vision in parts of the spectrum we cannot see.

    Many bird species have four vision pigments:
    Hart NS, Partridge JC, Bennett ATD and Cuthill IC (2000) Visual pigments, cone oil droplets and ocular media in four species of estrildid finch. Journal of Comparative Physiology A186 (7-8): 681-694.

    Bees have UV vision - the main reason postulated is that flower petals as seen in UV look very different from the way we see them. They seem to have "landing lines" in UV photos of showy flowered, bee-pollinated species. This is my personal observation.

    So you might ask: why do humans have only 3 pigments? Rather than why does it work?

    The short answer is processing power. We do not really 'see' color, rather, we infer color.

    This was mentioned in earlier posts in a different way.
  19. Nov 25, 2012 #18


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