Why is the sky blue ?

Discussion in 'Earth Science' started by Zarkov, Sep 1, 2003.

  1. Zarkov Banned Banned

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    well why is it ?

    I warn you you had better have a good argument.

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  3. MRC_Hans Skeptic Registered Senior Member

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    We've been through all that before, but OK, I'll bite: Scattered sunlight. Mostly from water molecules, I understand.

    But I'm sure you'd have an alternative thesis, Zarkov?

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    Hans
     
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  5. James R Just this guy, you know? Staff Member

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    Rayleigh scattering.
     
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  7. (Q) Encephaloid Martini Valued Senior Member

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    Light interacts with the nitrogen/oxygen molecules in our atmosphere and 'scatters' the higher wavelengths of visible light - higher wavelengths scatter more than lower wavelengths.
     
  8. Zarkov Banned Banned

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    Consensus... scattering

    JR mentioned Rayleigh scattering, What is this ?

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  9. Leapy Registered Member

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    Yeah. Like mirages are caused by light refractions, with the sky it is just the reverse, i.e. light from the sea is seen in the sky, thus the sky appears blue.
     
  10. James R Just this guy, you know? Staff Member

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    Sorry Leapy, but that's not right. It is quite a common misconception, though.
     
  11. eburacum45 Valued Senior Member

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    (That is like the explanation I have heard given for the Northern Lights- it is supposed to be 'reflections off the ice'.)

    All explanations involving reflection are not correct.
    Rayleigh Scattering it is.
    (oops, that's not quite right)
    ( the photons are technically being reflected off the individual scattering particles I suppose)
     
    Last edited: Sep 3, 2003
  12. Zarkov Banned Banned

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    >> Rayleigh Scattering it is.

    Got any examples of this effect ??

    What sized particles are involved ?

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  13. guthrie paradox generator Registered Senior Member

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    Are you asking a trick question?

    Its due to the molecules, not particles, though you likley get some scattering off the various particles in the atmosphere, look at the odd coloured sun and moon you can get in heavily polluted areas.
     
  14. Zarkov Banned Banned

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    >> Are you asking a trick question?

    mmmh, I am of the opinion that some of concepts involved in Rayleigh scattering are erroneous... namely the SIZE of the ?? particles ( you refer to them as molecules ).

    So what are real labatory examples of Rayleigh scattering.
    If it workd it must be able to be demonstrated... not just claimed!!

    I think the blue sky is NOT due to current explanations.

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  15. guthrie paradox generator Registered Senior Member

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    Molecules are made up of atoms, ie atoms joined by bonds involving electrons. The light wves are allegedly scattered by the molecular bonds, i think, but are scattered at different angles due to different frequencies. Eburacum45 gave a good link to explanations.

    Tell you what, you tell us whats wrong, and dont just ask us to prove it using laboratory experiments, that gets dull, because most of us dont have access to the kind of laboratory needed to do that, moreover, its the kind of questioning that Leeaus does, and gets us all no where.

    ah ha, google to the rescue:
    http://newton.uor.edu/facultyfolder/deweerd/research/scattering-AJP.pdf

    anyhow, my physics is rather rusty. maybe someone else can explain better.
     
  16. Zarkov Banned Banned

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    Ok, Rayleigh scattering produces red light with transmitted light and blue light by reflected light... ie a light source viewed through the medium applears red and that viewed at an angle to the light source appears blue.

    This is easily seen, when light illuminated smoke. We see a red Sun, and otherwise, blue smoke.

    This is classic Rayleigh scattering, except Rayleigh scattering is supposedly peculiar to oxygen gas, and nowdays is attributed to nitrogen (because oxygen and nitrogen gasses have similar molecular distances,.

    The size of smoke particles are magnitudes greater than gas molecules.

    >

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  17. MRC_Hans Skeptic Registered Senior Member

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    Why are we not surprised?

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    SO, Zarkov, what is YOUR explanation? :bugeye:

    Hans
     
  18. Zarkov Banned Banned

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    Nah, first you prove the established reasons.

    They can not be demonstrated to be correct, the maths relies on assumptions that are not born out in reality.

    Clue: What colour is water ?

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  19. river-wind Valued Senior Member

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    water is a clear liquid, with very small amount of color to it, due to a combination of the absorbtion of certain wavelenthgs of IR radiation and the neutralising effect of H-bonding between molecules. Why do you ask?


    http://webexhibits.org/causesofcolor/5B.html
     
    Last edited: Sep 5, 2003
  20. Zarkov Banned Banned

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    >> ater is a clear liquid, with very small amount of color to it,

    What colour, and can this colour be seen with the eye ??

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  21. Nasor Valued Senior Member

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    If you want to see Rayleigh scattering in action, shine a laser into something that scatters light well (like carbon tetrachloride) and set up a photomultiplier tube with an attached motorized grating monochromator perpendicular to the path of the beam. You can scan over all of the wavelengths around your laser and see the affects of all types of scattering, including Rayleigh scattering. Usually this sort of thing is done to look at the more interesting types of scattering, like Raman scattering, but the Rayleigh-scattered light will show up just fine.
     
  22. Nasor Valued Senior Member

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    Water is blue because water molecules absorb red light when their bonds stretch. The lack of red light makes the water appear blue. This is different from Rayleigh scattering. Rayleigh scattering is caused by oscillations in the electric field around the molecule, not stretching of the hydrogen-oxygen bonds within the molecule.
     
  23. eburacum45 Valued Senior Member

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    I can see your problem; but Rayleigh scattering is dependent on the size of the particle (which has to be smaller that the wavelength of the light) and the distance between the particles;
    too close and the scattered slight will recombine.
    from here
    http://www.sundog.clara.co.uk/atoptics/sunsets.htm#rayleigh

    The reds, yellows and golds arise because throughout the long path length, the air itself *, small dust and aerosol particles (Rayleigh scatterers**) scatter short wavelength blue and green rays much more strongly than longer wavelength yellow and red . The remaining direct unscattered light is dimmed but relatively enriched in reds and yellows. Magnificent sunsets occur when the upper atmosphere contains extra fine dust from a volcanic eruption. Absorption of specific green and blue wavelengths by ozone and water vapour molecules redden the light further.

    Large dust particles and suspended water droplets scatter light differently (Mie scatterers***) and do not produce vivid red sunsets, they merely dim the sun.


    *Scattering by the air
    Air molecules in the low density upper atmosphere behave as Rayleigh scatterers. However, closer to the ground the air is too dense for Rayleigh scattering to be significant because waves from the closely adjacent scattering molecules overlap and destructively interfere in all but the forward direction. In the middle atmosphere random molecular motion and collisions produce momentary density fluctuations that scatter the sunlight. The effect, first described by Einstein and Smoluchowski, is similar to, but not the same as Rayleigh scattering.

    **Rayleigh scatterers
    Particles much smaller than wavelengths of light scatter light in all directions. Their scattering is inversely proportional to the fourth power of the wavelength. Blue (~450 nanometer wavelength) is scattered over four times more strongly than red (~650 nm). Small dust particles are Rayleigh scatterers. Some smoke particles are small enough also, watch smoke from a fire, it looks red or brown when viewed against a bright light but blue/white otherwise.

    To me, this explains why the blueness of the sky appears to be high above our heads; it is only in the upper atmosphere that Rayleigh scattering is visible.
    _________________
    SF worldbuilding at
    http://www.orionsarm.com/main.html
     
    Last edited: Sep 6, 2003

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