Why don't nearby atmosphere look blue.

  1. Hi all

    We know white sun light consist of seven colours and that when it passes through the atmosphere then due atmosphere it scatters. Amount of scattering is more for shorter wavelengths( maximum for voilet). Due to multiple scattering the effect of voilet scattering washes away and blue scattering dominates at large distance.

    The atmospheric particles absorbing blue light radiate it ( after some time) in all,directions( this is what scattering is, right?). Text says this scattered blue light reach our eyes and therefore sky appear blue to us(when we look up or at far open distance).

    My question is that if particles ( gas molecules) scatters blue light, why don't we see every thing(empty space between my room window and the tree in the lawn) blue, ( after all it is atmosphere only between my window and tree) rather the atmosphere in between appear some what pale yellow. Why only atmosphere of above height(I.e. Sky) appear blue.

    Regards
     
    Last edited: Jan 29, 2014
  2. jcsd
  3. A.T.

    A.T. 5,621
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    There is not enough air on this short distance for the effect to be noticeable. Distant mountains appear blue.
     

  4. Thanks A.T. for your reply, but the light(blue) has already been scattered, from huge atmosphere(from where it is comming).

    It does not make any sense that the scatterers must be far away to observe the scattering taking place( I don't know what I am missing still).
     
  5. sophiecentaur

    sophiecentaur 13,565
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    You are missing the fact that the nearby atmosphere is scattering blue light. It's just that there are 100km+ of more distant atmosphere in the same direction which are contributing equal amounts (per metre of path).
    Also, you need to look again at the 'Blue' sky. It is only 'Bluish'.

    If you can get hold of a photo of a scene with the bluest sky you can find and look, with a photo-processing program, at the actual RGB values in that blue patch. If the Blue were saturated, R and G values would be zero. They are far from zero.

    We can be very sensitive to some de-saturated shades. Do the same RGB test on pictures of faces, they may appear very different 'colours' but you will find that the ratios of RG and B differ by only a small amount. This is true, even for pale Nordic skins and dark African skins - much less difference than you could ever imagine; it's the luminance that is different, rather than the chrominance because the dark pigment is pretty much a neutral grey..
     

  6. I will highly appreciate if you eleborate

    1. why do we need a thick layer (100km +) of atmosphere to observe scattering?

    2. When the upper atmosphere already scatter the blue light(which needs to reach our eye here on earth, and is reaching also), how come this blue light changes to yellow or white again?

    3. Since the blue light(after scattering) reaching to us(when we look up the sky) it means the lower atmosphere is not hindering the blue light to reach our eye, then why it is not blue in lower atmosphere as in the sky dispite the fact that blue light is reaching the earth?
     
    Last edited: Jan 29, 2014
  7. D H

    Staff: Mentor

    What you are missing is distance.

    Rayleigh scattering is a low probability event. A clear sky is almost like a vacuum to visible light. The mean free path between scattering events is in the tens of kilometers for blue light and is several hundred kilometers for red light. When you look across a room, the light that is reflected off the far wall essentially is subject to no scattering whatsoever. The distances have to be large (multiple kilometers) before you start seeing scattering effects.
     
  8. sophiecentaur

    sophiecentaur 13,565
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    If there were no atmosphere, we would see no scattered light. Hence the pictures in space have a black background. Very high flying aircraft see less scattering and the effect gets progressively less with altitude. The light that you see consists of all wavelengths - just more blue than the longer ones. Go to a distance of, say 100km and you will see a pretty dark sky but it will be detectably 'blue' because the proportions are pretty much the same (slightly different because the gas mix changes with altitude). On the surface, we see the maximum of scattered light,

    The blue light doesn't 'change colour' to white of yellow, with altitude.
    If you consider the light in the direct path between Sun and your eye, the more atmosphere it passes through, the bigger proportion of blue light is lost (scattered away) and what is left will be progressively 'redder and redder'. Hence the Sun looks reddish when lower in the sky because its light is passing through a longer path through the atmosphere. What has been lost from that reddish light that we are seeing is visible to other people as a blue sky.
    As D H says, scattering is a rare probability event so the scattered light that we see from a blue sky is a small fraction of the Sun's brightness (say 1 millionth). Some if this will be scattered again, of course (but 1 millionth of 1 millionth is just not detectable)
     

  9. Many of my doubts about scattering are getting cleared due to you guys, and now I understand how distance(continues scattering over long distance) is important to,observe an appreciable amount of scattering.

    But one thing more I want to ask to make things crystal clear :

    I understand for appericiable scattering to take place distance should be large and that the nearby atmosphere(due to not sufficent thickness or distance from our eye) is not able to do enough scattering. Now my question becomes, why do I need to look at distant atmosphere( Sky I.e. Why do I need to look towards that direction) to observe the scattered blue light, while the fact is that light from any direction during the day is the same scattered light which is in sky above) When the long distance atmosphere is successful in scattering the blue light(upto earth) I should be able to observe it at any angle, whereever light of sun(after scattering from long distance atmosphere) is reaching.

    This logic of mine urges me to have blue even in nearby atmosphere, while at the same time I agree with D H, that lower atmosphere(itself) could not provide sufficient scattering to produce blue.

    Sorry for my less understanding and weak mind which is slow in learning.
     
    Last edited: Jan 29, 2014
  10. sophiecentaur

    sophiecentaur 13,565
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    The sky looks pretty much blue in all directions - except directly at the Sun. Even when directly overhead, the Sun will not look exactly the same colour of 'white' that it will appear in space.
    This only applies to a perfectly clear sky. Clouds of water droplets (and ice crystals) and dust can change the situation. You very seldom get perfect atmospheric conditions to support what I am saying.

    You can do a simple experiment (a scale model of the atmosphere) in a dark room with a very few drops of milk added to water. A lamp or white OH projector beam, viewed directly through the cloudy, scattering, water will look redder and you will see a bluish haze in the water from the side. The walls need to be low reflectivity (blackout fabric preferably), if you want a good result. Being so much more dense than the air, the effect shows with only as few tens of cm of cloudy water. A fish tank is quite good for this and the water cloudiness needs to be barely detectable.
     
  11. ***scatterred away*** ....***left*****

    sopiecentanur I think a part of my confusion is that what really happens after scattering does the coloura filter out(as you say sactter away) as they pass and only unscattered liight could cross the curtains(after scattering) of atmosphere and each time a colour scatter away the white light loses its colours starting from blue(or voilet)? If this is the case how can we say blue sky when blue has filtered out and is not reaching the earth(in our eye).

    And another case may be scattering may mean that colours just came in visibilty as they scatter. Each time when when an particular colour scattered by sufficient amount we can observe it. But why do we need to look at a particular angle(I.e. towards sky) to observe the scattered blue light(scattered by long distance atmosphere) as I questioned in prevous post.

    Thanks a Bunch
     
    Last edited: Jan 29, 2014
  12. A.T.

    A.T. 5,621
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  13. sophiecentaur

    sophiecentaur 13,565
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    Take a beam of sunlight that is passing through the atmosphere 50km over your head. You will never see that beam. All you will see is a tiny portion of light that's been scattered out of that beam and in your direction. The spectrum of that scattered light is slightly tilted up on the short wavelength direction so it looks bluish. The 'colour' of the scattered light (i.e. relates to its spectrum) is always the same. As I said before, the blue from the 'blue sky' may be scattered but that's only 1/millionth of the millionth that came from the Sun (in any particular direction) The secondary scattering you refer to is just not relevant.


    The only 'direct' light you will see from the sun comes in a straight line from the Sun to you. (That's obvious but you need to bear that in mind.) You only see the sun at one spot in the sky. All the rest is scattered light.

    Have you read the wikipedia article on 'why is the sky blue'?
     
  14. sophiecentaur

    sophiecentaur 13,565
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  15. FactChecker

    FactChecker 707
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    My 2 cents:
    The scattered light from the sky is the blue component of light that otherwise would not reach your eye. The rest goes right on through to space. So you see more blue from the sky than other colors. Sunset is what remains after the blue has been scattered away and the remaining light clipped the earth and didn't quite go through to space. Suppose you look at something on the ground. Some blue light from it is scattered away from your eyes. Based on that alone, it should look less blue. But that missing blue is replaced by scattered blue light from other objects near it. So it evens out. The mind compensates for any small differences and you do not notice them.
     
    Last edited: Jan 29, 2014
  16. sophiecentaur

    sophiecentaur 13,565
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    Be careful with your words here. Light of all wavelengths is scattered; it's just that more blue than red etc. is scattered. Your computer RGB values will show you that, even if you don't like the theory.
     
  17. FactChecker

    FactChecker 707
    Gold Member

    You made a good point that when you look at the sky away from the sun it is primarily the scattered light that you see, whereas looking directly at the sun shows you more of the original light (although some higher frequency light has been scattered.) However, that doesn't address the original question of why that does not appear to happen to light from other objects. The computer RGB values only measure, not explain. I was trying to explain without introducing unnecessary complications.
     
  18. sophiecentaur

    sophiecentaur 13,565
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    It does. If you take a photograph of something in shade (no direct sunlight) it looks a different colour from how it looks in the sun. (White Balance) Different but not very different (more RGB values would show the amount) because the light from the sky is pretty much White - with a strong hint of Blue. All digital cameras, these days, have an auto colour balance setting that adjusts the RGB gains so that the whole picture balances out to an average 'grey', helping to reduce the colour balance problem. Old film cameras were much more 'honest' and a roll of film gives a vast range of reproductions of someone's face because it just says what it sees.

    The problem is that the intuition about colour vision (even down to the "seven colours" in the OP) is so strong that there are a lot of misconceptions that need to be dispelled before anyone can get a proper grasp.
     
  19. Ok so it means we just see in sky(in general other than the case of watching sun directly) the scattered light only, this now makes me well understood as to why actually sky is blue, i read wikipedia also.

    Now let me put what I understand.

    Both scattered and unscattered(white) light is reaching the earth and are stricking the particles of(lower atmosphere) to view atmosphere(with whatever colour) light again have to scatter from these particles of lower atmosphere. Thing is that the scattering of the particles of lower atmosphere is not sufficient enough(due to short distance between them and us) to get blue in amount to reach our eye. And almost unscattered light(from lower atmosphere) is reaching our eye when look at lower atmosphere.

    Is this what you people are saying? Is my interpretation Ok?

    Thanks for your continues support
     
    Last edited: Jan 30, 2014
  20. FactChecker

    FactChecker 707
    Gold Member

    The blue tint of a photograph taken in "open shade" is the opposite effect that would be expected when the blue light from an object is scattered. When the blue is scattered, one would expect a red tint in the remaining direct light, sort of like a sunset, but not so extreme. The point I am trying to make is that the scattered blue light from other objects can make up for the reduction of blue directly from the object you are looking at. In fact, the case of open shade, there is such a large component of scattered blue and so little direct light, that the overall effect is a blue tint to everything. The effect is clear and measurable "even if you don't like the theory".
     
  21. sophiecentaur

    sophiecentaur 13,565
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    What do you mean by that? The only unscattered light that gets to you is directly from the Sun. All the other light from the 'sky' has been scattered.

    The effect of secondary scattering (you seem to keep ignoring this) is absolutely minimal and not part of this explanation. (1/1,000,000 X 1/1,000,000 = Nothing)

    Could you draw a picture showing where you think this "un-scattered light from the atmosphere" is supposed to come into existence? It either goes directly in a straight line from the Sun or it is scattered.
     
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