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Mean colour of visible spectrum?

  1. Jan 20, 2015 #1
    How the Mean colour of visible spectrum is yellow?
    Shouldn't
    that be green according to acronym VIBGYOR?
     
  2. jcsd
  3. Jan 20, 2015 #2

    Quantum Defect

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    This depends upon the light source, and how you calculate the "mean", I think.
     
  4. Jan 20, 2015 #3

    Drakkith

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    I've never heard this before. Can you post a reference? It may help to get some context.
     
  5. Jan 20, 2015 #4

    DaveC426913

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    The mean is going to be based on wavelength - which ranges between 350 and 800 nm.

    It depends on what source you use for "visible light", as there seems to be a quite a range of opinions.
     
  6. Jan 20, 2015 #5

    mathman

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  7. Jan 20, 2015 #6

    DaveC426913

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    Sure but
    a] question was not about solar spectrum, just visible spectrum, and
    b] question was not about average (i.e. weighted), it is about mean (i.e. highest minus lowest).

    In this example, mean is 600nm
    visible-spectrums.png
     
  8. Jan 20, 2015 #7
    There was a question in my practical manual-
    " In general for which colour we take the refractive index of a material in lens and glass slabs.""
    The answer was given
    Yellow colour. Since it is the mean colour of visible spectrum.

    I had a doubt as green colour lies in between the visible spectrum. The mean wavelength and mean frequency of visible spectrum should be green?
     
  9. Jan 21, 2015 #8
    I've understood the acronym as ROY G BIV
     
  10. Jan 21, 2015 #9

    jbriggs444

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    The statement was vague. The boundaries of the visible spectrum are not exact. The statement was not about mean wavelength or mean frequency but about mean "color". All averages are weighted. It's just that the weights are often assumed to be uniform. But uniform by what measure? A uniform weight by frequency will give a different mean than a uniform weight by wavelength.
     
  11. Jan 21, 2015 #10

    DaveC426913

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    Either way it's a fabrication.

    Newton saw 6 colours but he felt strongly that 7 was a divine number, so he added indigo.

    I will see if I can find a reliable reference for this. There're plenty of not-so-reliable references to it.

    "It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. "
    http://colourware.org/2009/07/20/indigo-a-colour-of-the-rainbow/

    "Newton probably had other, very good reasons to define the Rainbow as a function of the favored magical number of seven,..."
    http://naturalmagickshop.com/articles/The-Myth-Magic-and-Science-of-the-Rainbow.html

    Here's one in the American Journal of Physics:

    "The author hypothesizes that Newton saw seven reasonably distinct colors in the artist's paint mixture color circle (red, orange, yellow, green, blue, violet, and purple) and therefore assumed he could also see seven distinct colors in his crude spectral projections."
    http://scitation.aip.org/content/aapt/journal/ajp/40/4/10.1119/1.1986607
     
    Last edited: Jan 21, 2015
  12. Jan 21, 2015 #11

    Drakkith

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    Violet wavelength - about 400 nm
    Red Wavelength - about 650 nm
    Mean Wavelength - 525 nm

    Light with a wavelength of 525 nm lies in the green area.

    I would guess that the the standard for measuring the refractive index using yellow light is due to historical reasons, probably something to do with the sodium spectral line at 589 nm.
     
  13. Jan 22, 2015 #12
    Okay, got it . Thanks to all. Sorry for asking a last off topic question but isn't that avatar of you Drakkith is a Doom game hero? I really liked that game in my childhood
     
  14. Jan 22, 2015 #13

    Drakkith

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    That it is!
     
  15. Jan 22, 2015 #14

    sophiecentaur

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    Why consider the mean wavelength when you could just as easily consider the mean frequency? Would you get the same answer? :) (Harmonic mean)
     
  16. Jan 22, 2015 #15
    I think we usually take arithmetic mean in these cases.
    Then both have same mean wavelength and mean frequency.
     
  17. Jan 23, 2015 #16
    Mean and [arithmetic] average are the same thing. Where are you getting "highest minus lowest" from? That's not any form of averaging as far as I know. If you take the average of 100 and 1 with that method, it comes out to 99, which doesn't make any sense.
     
  18. Jan 23, 2015 #17

    sophiecentaur

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    Arithmetic mean of what? It is only arbitrary and historical that we measure the wavelength of light in preference to the frequency. In fact, the Chemistry of what goes on in our eye receptors will be frequency based and not wavelength based. (i.e. photon energies)
    You should try with some different random values before you make a statement like that.
    In general, the harmonic mean is not the same as the mean of a set of numbers.
    (A +B)/2 is not the same as 1/((1/A + 1/B)/2), which is what you are claiming.
     
  19. Jan 23, 2015 #18

    sophiecentaur

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    This thread is, as is very common, mixing up the notion of Colour with Wavelength. Our Eyes are not Spectrometers and they (plus brain) see colours, which are usually wavelength combinations. We use just three colour sensors which have very broad overlapping responses. The 'colour' we perceive has nothing necessarily directly to do with the mean or peak of the spectrum of the light. The tristimulus system does work on a mean or centre of gravity of colours on the CIE chart but that is a two dimensional display and not a one dimensional spectrum.
    We do not 'see green' when there is a peak in the spectrum in the region of 'spectral green' because we are not designed to. That's all there is to say about it, unless you want to dig much deeper into the whole business of colour perception. It is just not that simple.
     
  20. Jan 23, 2015 #19
    I know arithmetic mean is different from harmonic mean.
    I am not claiming that both are equal.
    I was kind of asking that why we have to take harmonic mean instead of arithmetic mean when we have to find mean frequency of visible spectrum?
    I thought till Drakkith's reply I was understanding most of the things.
    He gave also the reason that yellow might be the mean colour considered because of sodium history.
    So if all in reality is frequency based according to your quote
    Then why harmonic mean?
    Isn't when we talk about means we usually refer arithmetic mean the most common?
     
    Last edited: Jan 23, 2015
  21. Jan 23, 2015 #20

    sophiecentaur

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    The frequency is inversely proportional to the wavelength so, if you want the same answer for both, you need to take the harmonic mean for one and the arithmetic mean for the other. I was originally making the point that using a mean wavelength is an arbitrary choice.
    Also, there really is no such thing as a Mean Colour (a very fuzzy quantity, at best). There is no future in a conversation that tries to relate what we perceive to the spectrum of incident light unless you are prepared to include how the three sensors will respond, separately, to the black body spectrum and then plot the resultant (processed) signal values onto a CIE chart. But you don't need to do that sum, to know the answer and that is - You Won't See Green. Every day you do that experiment when you look at sunlight reflected on white surfaces.
     
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