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Why does the colour wheel exist?

  1. Jun 22, 2011 #1
    A picture that seems to show up a lot in high school physics is one like this:

    [PLAIN]http://www.kollewin.com/EX/09-15-03/electromagnetic-spectrum.jpg [Broken]

    There's the wide range of electromagnetic waves at different frequencies, of which we can only see a narrow band. My question is, why do red and purple seem to overlap so nicely? Why, for example, can we make things like below, where we go through the rainbow from red to purple and then back to red again?

    [URL]http://www.color-wheel-pro.com/pics/color-wheel1.jpg[/URL]

    From looking at the electromagnetic wave spectrum, it seems like there's no reason for purple to blend nicely with red at all, and yet it seems to in practice. Is there a good reason for this? What would happen with animals that can see different parts of the spectrum than us (like ultraviolet, for example)?
     
    Last edited by a moderator: May 5, 2017
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  3. Jun 22, 2011 #2
    I suppose it's a bit subjective really isn't it? If we were unable to see violet, then you might well ask why indigo seems to blend in so well with red. Personally I think that any colour can blend in nicely with another as long as there's transition between the two.

    When it comes to perception which is outside our visual range, then I'd imagine the same sort of conditions would apply to the observer. Whether they'd appreciate the aesthetic of the blending of perceptible wavelengths is open to question.. I'll stick my cat betwixt a sun-lamp and rainbow wheel later on to see how she reacts :)
     
    Last edited: Jun 22, 2011
  4. Jun 22, 2011 #3
    The color wheel has to do with vision more than the physical properties of light. There are only 3 color receptors in the retina so it isn't a perfect reflection of the wavelength spectrum. (for example you can trick your eye into seeing yellow with light that is actually a combination of red and green light in the right proportions.)

    As it's usually drawn, primary colors across from each other on the color wheel are the complementary colors that show color aftereffect. This is the effect that if you stare at a green square that quickly changes to white, you'll see the square as a violet color. This has to do with saturation of the green photoreceptors. When you stare at the green square they adapt, but the other two types of color receptors (blue and red) do not. So the violet color you see is a result of the stimulation of the other two relative to the green. That's why it goes from blue to violet to red - violet is the color we see when red and blue photoreceptors are stimulated at the same time.
     
  5. Jun 22, 2011 #4

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    Why does purple have a reddish tinge to it?
    As opposed to red that does not have a bluish tinge to it.

    Since it's on the other side of the spectrum, one would expect that the red photo receptor of the eye is stimulated the least by purple!
    Weird huh?

    Wouldn't this seem to suggest that the red photo receptor is also susceptible to purple?
    But I've never read anything like that....
     
  6. Jun 23, 2011 #5
    The color that seems to blend with red on that well is not the actual violet of the spectrum but rather a shade of magenta.
    I don't think that the actual violet end of the spectrum looks like it has red in it. I mean, if you look at a violet laser spot. It does not really look like the magenta shade on that well.
    The distortion may be intended or maybe comes from the color rendition on monitors. Some pictures of bright violet laser spots have this magenta hue too.
     
  7. Jun 23, 2011 #6

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    True enough!

    But why then is there violet beyond blue in the spectrum? :confused:
     
    Last edited: Jun 23, 2011
  8. Jun 23, 2011 #7

    DaveC426913

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    You know, this is a really good question. Why can we see violet at all?

    There are various flavours to the idea that we can see magenta (the complement of green), which is a combination of red and blue.

    But there is also a pure violet that is not a combination. Why can we see it? I guess because the blue receptor is sensitive to a range.

    Does that mean that when we see blue, we are seeing the effects of stimulation of blue receptors and green receptors? It would suggest that, if we eliminated the green receptors, so that we could only see what is stimulating the blue receptors, it would appear violet, not blue.

    Why does the blue receptor allow us to see violet beyond blue but the red receptor does not allow us to see some other colour beyond red?
     
  9. Jun 23, 2011 #8

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    An alternative explanation might be that our photo receptors are not nice band filters, but that they can be stimulated at other frequencies than their main frequency......

    Edit: perhaps a resonance frequency?
    I just looked it up: violet is 380–450 nm, while red is 620–750 nm.
    So twice the wavelength of violet overlaps the red range.....
     
  10. Jun 23, 2011 #9

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    I couldn't believe that when colors were inventoried and standardized this "problem" would not have been acknowledged.
    So I researched a bit how colors are standardized and found this wiki article:
    http://en.wikipedia.org/wiki/CIE_1931_color_space

    It suggests that an observer has a response curve for red photo receptors that has a second peak in the blue part of the spectrum.
    Btw, this is not at a resonancy frequency.
     
  11. Jun 24, 2011 #10

    Claude Bile

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    The answer is pretty simple, our brains have adapted to it. We have 3 colour receptors, one for red/green/blue; it just happens that our red receptors have a small response in the blue part of the spectrum.

    370-420 nm wavelength light excites primarily our blue receptors, plus a small amount of our red receptors, which our brain interprets as violet.

    I encourage people to look up the spectral response of our different colour receptors; it answers a fair deal of these questions, and is also quite fascinating.

    Claude.
     
  12. Jun 24, 2011 #11
    Well now, I was going to answer the same because I thought that was the case, but looked at the wiki version of the response curves and didn't see any blue-bump in the red receptor. In fact the green receptor response seems to be stronger than the red under and beyond the peak of the blue. Are those graphs not entirely to be believed? Or are we just hand-waving the violet-is-reddish-blue thing when it is "really" greenish-blue?
     
  13. Jun 24, 2011 #12
    Looking for real research data, I found some actual measurements done on individual or small groups of cones,for humans and monkeys.
    The paper on humans is
    Spectral sensitivity of human cone photoreceptors
    J. L. Schnapf*, T. W. Kraft & D. A. Baylor
    Nature 325, 439 - 441 (29 January 1987)
    The monkeys were done by the same group.

    It seems that the red receptors have a small deep in the green-blue region and then a slightly increased sensitivity at the shortest wavelength end.
    Unfortunately they did not measure the blue receptors.
     
  14. Jun 24, 2011 #13

    DaveC426913

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    I have always seen this kind of graph:

    540px-Cone-fundamentals-with-srgb-spectrum.svg.png
    http://en.wikipedia.org/wiki/Color_vision

    so I don't know about this 'responsive to blue' you mention.
     
  15. Jun 24, 2011 #14

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  16. Jun 24, 2011 #15
    I guess all they say about wikipedia is right...even their own entries don't agree, vis:
    http://en.wikipedia.org/wiki/CIE_1931_color_space
    http://en.wikipedia.org/wiki/Color_vision
    Don't believe anything you read there without independent corroboration.

    Unfortunately, "Spectral sensitivity of human cone photoreceptors" is behind the wall of scientific pay-for-play at Nature so us pleebes may never know the one-true-response curve.
    But thanks for the info!
     
  17. Jun 24, 2011 #16

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    I believe both wiki articles to be true.

    The Color_vision article describes the response of the 3 types of cones L, M, and S.
    The CIE article describes how people "observe" colors with standardized x, y, and z curves.
    These are different things.
    And it is not clear why there would be a discrepancy between the two.

    My 2 cents: perhaps the S-cones are cross-wired somehow to the L cones?

    Does anyone have access to the "Nature" article?
     
  18. Jun 24, 2011 #17
    http://books.google.ca/books?id=OcW...CDkQ6AEwAg#v=onepage&q=vision science&f=false

    Here is a diagram of spectral sensitivities of cones from a book which is accessible on Google books. The page is 157.
    Very interesting topic. I did not think about it too much. Just considered the Wikipedia type graphs with the two bell shaped curves.
    The secondary maximum of red receptors does not seem to be confirmed by real measurements. Maybe is a subjective maximum, not a physical one.
     
  19. Jun 24, 2011 #18

    Drakkith

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    Well, I typed up about half of a long response, but realized the situation was a bit more complicated that I was trying to describe. As such I'll just simply say that I think the reason that violet looks red is because your "red" receptor is activated somewhat by the color, whereas when we see red light and lower in wavelength the blue receptor does NOT activate at all. Hence violet looks like a mixture of red and blue, and in fact that is how we make the color when mixing paints or pixels from a TV. In contrast, your Blue receptor can never see into the red part of the spectrum, so there is no similar effect for red.

    I would expect that if the Blue receptors overlapped the Red in a similar way then light that was a combination of the two could be perceived as violet as well depending on the amount and ratio of activation. Note that this is talking about monochrome light, as mixing paints still results in the perception of that specific wavelength range even though there is no light in the violet wavelength reaching your eye. A side effect in my opinion would be that a rainbow would go Violet, Red, Orange, Yellow, Green, Blue, Violet, making a "Color Wheel" with each color blending into the next in a circular fashion.
     
  20. Jun 24, 2011 #19

    DaveC426913

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    But it isn't.
     
  21. Jun 24, 2011 #20

    Drakkith

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    How so? Everything I looked at here and elsewhere showed that the red receptor does have a response range in the violet wavelength. Did I miss something?
     
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