1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is the sky blue?

  1. May 14, 2014 #1
    I was practicing for my upcoming optics exams, by making an old exam.
    And got stuck on some problems that seem so silly.

    1. Why is the sky blue?

    My answer:
    To answer this problem I first need to mention Rayleigh Scattering, a form of scattering which has a very large wavelength dependance. Only light with a wave length that is about 10% of the size of the particle gets scattered.
    White light from the sun must travel trough the atmosphere, the atmosphere contains lots of particles.
    Most of these are very small, so only the blue and violet rays from the sunlight can get scattered on these molecules. This scattering from the blue waves, makes the sky almost everywhere you look seem blue.

    (This how i would explain it. Does anyone know if this is correct/complete?)

    2. Why is the sky near the horizon white/pale?

    My answer:
    If you look at the sky near the horizon, you are looking through way more atmosphere than when you are looking at the sky above you. particles are scattered multiple times, eventually reds and greens get almost as strong as blue, thats why sky is whiter?

    (not sure why and if this is true? why do green and red eventually get as strong as blues? why does it matter if light scatters multiple times?)

    i'm especially stuck on 2, not sure why the sky is whiter near the horizon.


  2. jcsd
  3. May 14, 2014 #2
    I think your explanation is OK.

    Why is the sky red at sunset time?
  4. May 14, 2014 #3


    User Avatar

    Staff: Mentor

    Have you tried putting that into a search engine?
  5. May 14, 2014 #4
    sure i have

    this is something i found

    As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.


    but still don't get it, sorry
  6. May 14, 2014 #5
    i would think that this would make the sky appear more reddish, since less blue is reaching your eye
  7. May 14, 2014 #6
    I think i get this problem:

    if the sun is going down it has to pass trough a thicker layer of atmosphere.
    The blue light is almost scattered 'out' when it reaches our eyes, while the green and the red are not yet.
    The most red is left(longest wavelength), so the sky appears reddish.
    (a mix of mostly red, little green, very little blue)

    ahhww, maybe this is not explained to good. Do you know if this is ok?
  8. May 14, 2014 #7


    User Avatar

    Staff: Mentor

    Google comes up with more complex explanations than that one.
  9. May 14, 2014 #8


    User Avatar

    Staff: Mentor

    That is a good explanation of why the disc of the sun appears red. But it falls short of explaining why the sky appears reddish. (Because, I think you'll agree, this looks like the same explanation for why the sky appears blue!)
  10. May 14, 2014 #9
    thanx, but why then does it appear reddish?
  11. May 14, 2014 #10
    yes, i know but the problem is that i still don't really get why the sky is paler near the horizon!
    i don't see how a thicker layer of atmosphere makes the light appear white.

    then light has to travel longer trough the atmosphere, so the blue light is almost scattered out, and red and green not? so i would assume here also the sky is reddish. I don't get the difference between sunset and the horizon problems.
  12. May 14, 2014 #11
    From what I remember from waves last semester, scattering is proportional to λ-4 so when the sun is glaring at you from a near 45 degree angle between the horizon, the scattering goes through enough particles so that only we only see the blue light since it has a small wavelength (or violet really, from what I understand our eyes normalize violet to blue because it's easier for them). During sunset, the sun is on the horizon so that path it has to take to get to our eyes is a lot smaller and it passes through a lot less particles. Because of that, the effect of the scattering is decreased and we see orange and red.

    The reason we see a paler sky near the horizon is because there is less scattering so more wavelengths are able to be seen. Thus white light.
  13. May 14, 2014 #12


    User Avatar
    Science Advisor
    Gold Member

    The distance through the atmosphere is a minimum when the sun is overhead; it is a maximum when the sun is at the horizon. Draw a picture - not to scale - and convince yourself.

    Also - Rayleigh scattering is not the only factor at sunset/sunrise - Mie scattering is also important because of the presence of dust/aerosols in the troposphere - these are minimized when you look directly overhead, but are maximized for a view at the horizon. Revise your earlier drawing to show the troposphere and stratosphere - just for the two views, and make it to proper scale.
  14. May 14, 2014 #13
    So the only way that makes sense then is if scattering makes wavelengths less visible, right? Highly scattered wavelengths are more prominent than less scattered ones? Red is seen at sunset because the blue light is scattered and we don't see it? I just had it completely reversed then right? lol. I had a feeling that was the case, that's why I tried to answer.
  15. May 14, 2014 #14


    User Avatar

    Staff: Mentor

    When the sky looks white anywhere it is just an issue of low transparency due to high humidity.
  16. May 14, 2014 #15


    User Avatar
    Science Advisor
    Gold Member

    The light which you "see" which carries an image - of the moon, a blade of grass, etc., is coming directly from that "object", subject only to reflections and refractions.

    Scattered light appears to be coming from "everywhere" within its scattering domain - hence the sun (which is white) appears yellow during the day because the blue is preferentially scattered ... and the sky appears blue during the day because the blue light is scattered "everywhere".

    At night the sky is clear - no more blue!

    When tiny water droplets are present the sky appears white - all of the light is being scattered, and we call this fog or clouds depending upon how close to the ground it is.

    As the sunset approaches the sun slowly changes color as more and more wavelengths are scattered. If you live on the coast you can see these effects clearly along the horizon.

    If you are patient, and perhaps lucky, you can even see the "green flash": http://mintaka.sdsu.edu/GF/

    I've only seen it once, during a sunset in Florida.
  17. May 15, 2014 #16
    As russ_waters said, the white color of horizon is caused bu humidity, dust and other particles in the atmosphere. The sky looks red on the sunset/sunrise because of the red light from Sun reflecting from the dust particles. That's all.
  18. May 15, 2014 #17


    User Avatar

    Staff: Mentor

  19. May 16, 2014 #18

    Claude Bile

    User Avatar
    Science Advisor

    One thing you might want to consider is the human eye response.

    In other words, why the sky looks blue rather than purple or UV!

  20. May 22, 2014 #19
    A complete explanation of the sky's color requires you solve the hard problem of consciousness...
  21. May 22, 2014 #20
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook