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Blue Sky/Red Sunset Confusion

  1. Jan 18, 2015 #1
    The sky is blue because blue light resonates with the air molecules and therefore has a higher scattering intensity (due to Raleigh scattering). Thus, we see blue light when we look at the sky (any light that we see, while not looking directly at the sun, must have been scattered).

    Is this right so far? If so, I am still confused by the following. Say that only red light and blue light are coming in. The red light is something like half as energetic as the blue light. How, then, can the blue light be scattered more than twice as intensely? That would seem to violate the conservation of energy. Is the answer that the rest of the red light is simply transmitted, and not scattered?

    Now, I still don't get why sunsets would be red. The sun is red because it emits red light most intensely, right? (It would be red if we looked at it from space). People constantly describe the sunset as red because the light has to pass through more atmosphere, so most of the blue light is "scattered out". Don't we still see that light? (We see both the scattered light and the transmitted sunlight, so we see blue+red+everything else?). I think this might be related to my second paragraph. Thanks!
  2. jcsd
  3. Jan 18, 2015 #2


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    Essentially, yes.

    What?? No. Sunlight is pretty much white.

    The sunset is red because all the blue light has been scattered away from your line of sight. The only thing reaching you is the unscattered red.
  4. Jan 18, 2015 #3


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    Sunlight is white light, in space. The blue light reaching your eyes at sunset is highly diluted, blue has been scattered in all directions, so only a small fraction of what was originally in the direct sun ray reaches your eye from that direction.
  5. Jan 19, 2015 #4


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    There's a few things that probably need clarifying here since they usually add to the confusion regarding the color of the Sun and sky.

    1. The Sun emits radiation most intensely in the yellow region of the spectrum. In other words, if we graph the power output across the spectrum, it would peak in the yellow region.
    2. Sunlight looks white in color. This is because of two main factors. The first is that the although the solar spectrum peaks in the yellow area, the difference between the the yellow and the other colors of the spectrum is relatively minor. The other factor is that we evolved to see visible light from the Sun as white. One could argue that if the solar spectrum were slightly different then we would see that combination of energy per frequency as white.
    3. The Sun's stellar classification is a yellow dwarf, but this is a misnomer. Stars with this classification range from 0.8 to 1.2 solar masses and vary from slightly yellowish to white in color.

    No, the blue light is indeed scattered much more strongly than the red light. This is because the wavelength of blue light is much shorter than red light, so it interacts much more strongly with air molecules, which are what do the scattering. Conservation of energy has nothing to do with this.

    See: http://en.wikipedia.org/wiki/Rayleigh_scattering#Reason_for_the_blue_color_of_the_sky
  6. Jan 19, 2015 #5
    I guess what I am trying to ask is the following: if the incident blue light has "power = 1" and the incident red light has "power =1," but the blue scatters with "power = 10," where does the extra energy needed to "amplify" the blue light come from?

    Why won't some of it scatter forward, towards you? Isn't that equally likely? It seems like, at most, the blue light is equally scattered over all angles?

  7. Jan 19, 2015 #6


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    Slight clarification: the red AROUND the sun is the backlit water in the atmosphere. During normal daylight it appears white, but backlit by the sun at sunset it is red.

    A good gauge of humidity is how dark blue the sky is. If it is very dark straight up and blue to the horizon in midday, it is dry. Lighter/whiter is more humid.
    Sure, but scattered equally in all directions means almost all is scattered away from you.
  8. Jan 19, 2015 #7


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    No, the blue might scatter with a power of, let's say, 0.8, leaving only 0.2 left to continue forwards.
  9. Jan 19, 2015 #8


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    Nothing gets 'amplified' all that happens is that some wavelengths are attenuated / scattered away more than others. Ever notice that it is less bright near sunset?
    At sunset, the light reaching your eyes has passed through many more km of it than at midday. On its way, the blue bits have been scattered preferentially - giving other people a blue (ish) sky and leaving you with a red (ish) sky.
    Note I use 'ish' to describe the light because it still has components at all wavelengths and the red and blue are actually pretty desaturated colours. Use a prism if you find that hard to believe - or even just look at the colour of your favourite T shirt at noon and dusk; it changes very little, showing that the liluminant is not actually very different..
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