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Sky colour explanation by scattering

  1. Feb 24, 2007 #1
    I have done a little research on the internet and I would like to ask my concept is correct or not.

    1) Sky is blue
    As the effect of rayleigh scattering is more effective for shorter wavelength, blue light scatter more than others (e.g. red). Furthermore, our eyes are more sensitive to blue than violet, so the sky is appeared to be blue.

    2) Sky is at sunset
    At sunset, the sun is far away and sunlight has to travel for a long distance to our eyes, most of the blue light scattered away and the intensity of blue light become very low, while red is less scattered, so red colour is dominant.

    3) space is dark
    As space is vacuum, no molecules responsble for scattering, so it is dark.

    4) cloud is white
    cloud contains large water molecules and mie scattering dominates. As mie scattering is not strongly dependent on waveleght, so the water molecules just scatter the white light of different wavelength in similar extent, so it is white.

    By the way, I would like to ask what is the meaning of "scattering is more effective" ? It means the light intensity is larger after scattering or the light scattering in a "wider direction" ?

    Thank you
  2. jcsd
  3. Feb 24, 2007 #2


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    Item 2 is somewhat in error. The distance to the sun is not a factor (compared to 1). The effect is primarily the result of the distance of sunlight through the atmosphere is greater at sunset or sunrise than at noon.
  4. Feb 24, 2007 #3
    I think "scattering is more effective" means the shorter wavelengh light beams will scatter more than the longer ones. You can see the color of the sky is due to the scattering phenomenon. When a light beam scatters, some of it changes to many directions and the rest still goes in a straight line. The scattering lights is what you can see in the sky
  5. Feb 25, 2007 #4
    Feynman's Lectures Book one has an excellent discussion of this subject. See pages 32-6 to page 32-9. You can find this book in most good libraries.
  6. Feb 25, 2007 #5

    Claude Bile

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    For 3) The sky appears dark simply because the quantity of light being scattered in the atmosphere has been vastly reduced, not because the scattering centres (molecules) have been removed.
    I would venture that the "scattering more effective" means a greater proportion of incident light is scattered.

  7. Feb 25, 2007 #6


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    I don't know what violet has to do with anything.
  8. Feb 26, 2007 #7
    Thank you for the explanation and correction!

    >>I don't know what violet has to do with anything.
    If scattering is more effective for shorter wavelength, violet is scattered more than blue and the sky should be violet.

    As for "scattering more effective", is greater proportion is scattered means greater intensity is scattered ?
    That means for shorter wavelegth, the incident light will evenly distributed in all directions and for longer wavelegth, the intensity of light in the incident direction is larger than other directions ?
    Last edited: Feb 26, 2007
  9. Feb 26, 2007 #8


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    The sky color is a mixture of many colors. Furthermore, the sun spectrum is not flat- I believe its peak is the yellow. Therefore the blue intensity is greater than violet. Finally our eye sensitivity is not unform over all colors.
  10. Feb 26, 2007 #9
    Isn't the red light refracted less than the blue light?
    Blue light hitting the atmosphere is refracted downwards while the red light can follow a straighter path.
  11. Feb 26, 2007 #10

    Claude Bile

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    Nope, what I said has nothing to do with intensity, I'm talking about the overall quantity of light. Think of it as a scattering efficiency Scattered Light/Non-scattered light.

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