Why is the sky blue and not purple?

  1. how is the sky blue and not purple? how come purple light is not scattered , or how come the sky does not appear purple to us .
  2. jcsd
  3. D H

    Staff: Mentor

  4. so violet light is scattered our eyes just dont pick it up.
  5. uart

    uart 2,776
    Science Advisor

    The whole range of visible frequencies are scattered, from red through to violet. However with more scattering happening at the blue-violet end and less at the red-orange end our eyes perceive this mixture of colors as blue.
  6. i see
  7. In fact birds see the sky in ultraviolet because their eyes have evolved that way.

    Food for thought.
  8. The "blue" in blue sky is attributed to Rayleigh scattering of sunlight. The frequency (wavelength) dependence is f4 (lambda-4). Purple is an admixture of red and blue. There is very little "red" in "blue" sky. Have you noticed that at 90 degrees to the sun, the blue sky light is nearly completely polarized (only when the air is cloudless and really pollution free). Why polarized?
  9. tiny-tim

    tiny-tim 26,041
    Science Advisor
    Homework Helper

    purple stripes

    Hi cragar! :smile:

    Only the stripes are purple :wink:
  10. blue is short wavelength light. red is long wavelength light. short wavelength light is scattered more.

    purple isnt in the rainbow
  11. uart

    uart 2,776
    Science Advisor

    I'm pretty sure that cragar actually meant violet even though he said purple, that was my interpretation of the question anyway. My understanding of what cragar was asking is basically : given that shorter wavelengths are the most readily scattered - and violet is the shortest wavelength we can see - then why don't we see the sky as violet.

    Actually it's interesting that although technically violet (the spectral color at around 380-420nm) and purple (typically a mixture of blue and red) are very different, in terms of human perception they are indeed very similar. Violet is necesarily represented as a shade of purple in RBG color-space on a computer monitor and it isn't all that bad an approximation of what the spectral color actually looks like (in terms of human perception). So in my opinion cragars use of the term purple, in the context of "how should the sky appear to us", really wasn't at all inappropriate (even if he was really referring to violet).
    Last edited: Jun 8, 2009
  12. D H

    Staff: Mentor

    Didn't you ever learn the mnemonic Roy G. Biv? The colors of the rainbow are Red, orange, yellow, Green, Blue, indigo, and violet. Isaac Newton thought the spectrum should have seven items to match the number of days in the year.

    Some rainbows:



    That's exactly how I interpreted his question. The question to be answered isn't "why is the sky blue?" cragar apparently knows the stock answer to this question as talks about scattering in the original post. He is questioning this stock answer, and this stock answer is only partially correct.

    The question to be answered is "If scattering is the sole answer to 'why is the sky blue', why isn't the sky indigo, or even violet? After all, indigo and violet should be scattered even more than is blue." The answer to this deeper question lies not only in the sky but also in our eyes and in the way in which we perceive color.

    When there is a lot of scattering the sky can appear to be indigo, or even violet.


  13. cepheid

    cepheid 5,190
    Staff Emeritus
    Science Advisor
    Gold Member

    Why is violet followed by even more green in both of the pictures of rainbows that D H posted?
  14. D H

    Staff: Mentor

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