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Why the sky is blue and not violet?

  1. Nov 21, 2014 #1

    kmm

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    I have some confusions about this that I'm not finding clear answers for. First, looking at the color violet, it looks like a sort of "purple". The answer as I've understood it to why the sky doesn't appear violet or this "purple" is that it has to do with a combination of the fact that there isn't as much violet produced by the sun and it's absorbed higher in the atmosphere. In addition to that is while violet appears sort of purple since it stimulates the blue cones and the red cones less, the net effect from the different frequencies of light coming into our eye is that the blue cones are stimulated most. So we see the sky as blue. What's confusing to me is that while thinking of violet as sort of a purple, I've come across some explanations that regard violet as a deep blue, not this "purplish" color. To add to this, if you look at a spectrum produced by a prism, it doesn't appear to show violet as this "purple" but a deep blue, however when you look at a rainbow you can see violet as this sort of purple. So I'm not sure what violet is actually perceived as. If violet is perceived as a purple, why doesn't this show up with a prism? If violet is perceived as a deep blue, why do we see "purple" where violet should be in a rainbow? Which is it?
     
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  3. Nov 21, 2014 #2

    A.T.

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  4. Nov 21, 2014 #3

    kmm

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  5. Nov 21, 2014 #4

    Andy Resnick

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    Violet/purple are not spectrally pure colors, but a product of our visual system (so are brown and pastel hues). Color perception allows a surprisingly rich analysis: for example, mapping the 'color line' (the usual spectral representation) to a 'color wheel' (the usual visual representation) involves a non-trivial mapping that introduces a singularity.
     
  6. Nov 21, 2014 #5
    All the rainbow colors are spectrally pure, but typical human vision has just three dimensions of color. The space of possible spectra is much larger than the space of colors that we can see, so many different spectra will give the same visible color. So purple (which is generally not spectrally pure) and violet (which may or may not be spectrally pure) look similar.
     
  7. Nov 21, 2014 #6

    cjl

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    I own a 405nm (violet) laser pointer, and I can tell you, I definitely perceive it as purplish, rather than blue. It does look a little different than a traditional "purple" though. It's kind of hard to describe without seeing it for yourself. That having been said, I wouldn't say it's just a deep blue.
     
  8. Nov 21, 2014 #7

    kmm

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    I don't quite understand this. I thought violet has a well defined wavelength range (380nm - 450nm). I understood purple to be a mixture of blue and red though. So I'm interested in how violet is actually perceived. In most depictions of violet, it looks like sort of a purple, I assumed because it stimulated blue cones and red cones. However, I don't see why a prism doesn't show this but a rainbow does. I assumed because a rainbow doesn't produce a pure spectrum.

    Are you implying that a prism is not spectrally pure? If you're saying violet looks similar to purple, which is what I'm trying to determine, then I have to wonder why this apparently is never seen with a prism.

    Interesting. I've read some other people saying the same thing. I wonder why the video said violet is perceived as deep blue then.
     
  9. Nov 21, 2014 #8

    A.T.

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    At around 1:10 min:

     
  10. Nov 21, 2014 #9

    kmm

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    OK now I understand.

    What I wonder now is, why is violet often portrayed as a "purplish" color? And as cjl and others I read have said, why does a 405nm laser pointer is perceived as a more purple?
     
  11. Nov 21, 2014 #10

    sophiecentaur

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    It's "portrayed" that way because you cannot "portray' spectral violet any other way than with a spectral source. Why would you be surprised that the eye is fooled / confused when presented with a very rarely seen colour? The 'violet' that's shown on film, TV displays and printing with a pigment is much the same as spectral violet - certainly near enough for that small range of colours on the bottom left of the CIE chart to be called violet. And it is only called violet. It's time to accept that classification can be very wooly and particularly when it doesn't really matter. Never, in human evolution, was spectral violet seen (the rainbow is far too desaturated to be classed as a set of spectral colours); we were not 'designed' to distinguish that colour. I very much doubt whether most readers of this thread have actually seen a good, spectral violet (the laser pointer thingy would be the only likely example).
     
  12. Nov 21, 2014 #11

    kmm

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    OK. I wasn't confused that the eye would be fooled when presented with violet, I just wasn't sure what the eye would actually perceive in that case. It's just that some people were saying deep blue and others a "purplish".

    On film and tv etc violet appears to be a "purplish" color. So in that case are you saying that is close to what spectral violet would be perceived as? Not a deep blue?
     
  13. Nov 21, 2014 #12

    cjl

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    I would say this image is pretty close to how I perceive my 405nm laser pointer (though my monitor can't quite replicate the true effect):
    http://www.ledmuseum.candlepower.us/netg405.jpg

    (Admittedly, with the caveats thrown in that the way you view that image will depend on your monitor and its calibration/settings).
     
  14. Nov 21, 2014 #13

    kmm

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    Ok interesting. This is something like what I would actually expect our eyes to perceive when presented with violet. Our cones aren't as sensitive to violet, so the red and blue cones would be stimulated yet stronger in the blue giving the illusion of this purplish color. Although, the video in post #8 seems to give a good explanation why this isn't actually the case. Maybe the 405nm laser pointer isn't pure 405nm light then.
     
    Last edited: Nov 21, 2014
  15. Nov 21, 2014 #14
    The 410 nm spectral line of Hydrogen certainly looks "purple" to me.
     
  16. Nov 21, 2014 #15

    Andy Resnick

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    A few comments- first, human color vision is based on three populations of cone cells:

    http://wearecolorblind.com/wp-content/uploads/graphs.jpg

    And thus, any 'pure spectral color' excites multiple cones. This is actually the reason why we can perceive so many hues, even though we only have (colloquially) 'red, green, and blue cones'.

    So deep blue light (380 - 450nm or so) excites multiple cones- and as you can see from the above graph, the red and blue cones respond more strongly than the green in that waveband, so that's why we describe purple as a mixture of blue and red.

    In any case, the more fundamental point is that 'color' as an attribute of electromagnetic radiation can be most easily quantified either as a linear scale or as a plane surface, and these different representations are not equivalent to each other. The first is used when quantifying the electromagnetic field in terms of it's frequency, while the latter is used to describe how color is perceived by the brain as a result of visual stimulation.

    http://en.wikipedia.org/wiki/CIE_1931_color_space

    It's not easy to describe 'saturation' in terms of the linear scale: all 'spectrally pure' light is maximally saturated, and mathematical descriptions of desaturated colors requires statistical models (correlation). That is, spectrally pure light is completely coherent, while desaturated colors (pastels, for example) are partially coherent and neutral grey is completely random.

    Does this make sense?
     
  17. Nov 21, 2014 #16

    kmm

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    Yes, I understand that. My only question at this point is how is violet perceived? "Purplish" or deep blue? Up to now I've seen that a prism shows it as a deep blue but others say they see violet lasers as a purple. As of now though, I'm inclined to say it looks a deep blue as in post #9. I'm suspecting that violet lasers may appear as a deep sort of purple due to some combination of an illusion due to color context and impurities in the light (not entirely composed of 405nm light say).
     
    Last edited: Nov 21, 2014
  18. Nov 22, 2014 #17

    sophiecentaur

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    I think you are trying for an answer that is far too definite, here. Colour is only a psychological response and it's 'learned' from experience and the general consensus. Historically, the rainbow was seen and the colours named, by popular opinion. I think that it is very likely to find that the appreciation of spectral violet will be different from observer to observer. That's not surprising as it is right on the band edge of our biological optical receivers. Looking at the far end of the rainbow and try to match it with a colour on a card or RGB display, many people will probably match it with a colour that's well inside the CIE curve. Their brains will be doing what all brains do and that is to find an answer to an unanswerable problem (i.e. matching two unequal things).
    There is confusion here because there really is confusion in our colour senses. the same thing even happens with the perception of white and greys and short term colour memory can give us all sorts of confusing colour sensations. If you accept that and don't demand a metrological answer then you can stop worrying about it.
    I guess that there will be specialist papers on 'The psychological perception of the colour Violet'. If you can find one, then you may get more sense than PF can yield. Personally, I reckon that anyone who's done a lot of work on colorimetry will not be inclined to investigate this problem because it is too wooly to nail down (as we have found here).
     
  19. Nov 22, 2014 #18

    kmm

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    I was starting to figure this, so I have to agree with you here. Thanks for all the responses.
     
  20. Nov 23, 2014 #19

    sophiecentaur

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    I had another thought about this 'violet confusion' and I think it may well be due to the fact that all three sensor responses are well down for spectral violet. (See Fig 1 in this link - and many others). Assessment of colour is made on the basis of the ratios of the three responses. As the 'blue' response curve is dropping fast with decreasing wavelength and the other two are dropping very slowly, the way that the ratios are changing will be subject to a lot of error (small - probably unmeasured - variations of the skirts of the response curves). The nervous system does the best it can with the three values it has to deal with but - for evolutionary reasons - it has not (needed to have) good discrimination in that region. We know that evolution seldom gives us abilities that we don't really need.
    Otoh, it is relevant and important to know about what is going on in the red regions of the spectrum, so the three analysis curves seem to be tailored much better to discriminate between reds and near- infra reds. Only yesterday, I was using a blow torch to heat a piece of steel up and I could detect a hint of 'red heat', without confusing it with a deep pink that a pigment could have produced.
     
  21. Nov 23, 2014 #20

    DrDu

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    An imo inportant point is also that the eyes sensitivity is much lower in the violet than in the blue.
     
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