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Why is air transparent?

  1. Jun 23, 2008 #1
    Why are the atmosphere's main elements ( Nitrogen and Oxygen) transparent to visible light.

    And what makes most gases have low absorption?
     
  2. jcsd
  3. Jun 23, 2008 #2

    Andy Resnick

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    Gases scatter little light becasue they have low density. The spectral location of absorption lines depends on allowed atomic (electronic, vibronic, rotational) transitions, and for Earth's atmosphere, our eyes have evolved to take advantage of spectral reagions with little scattering.
     
  4. Jun 24, 2008 #3
    I think Andy has really hit on the explanation here. I don't think it's an oddity that atmospheric gases don't have absorption lines in the visible spectrum but rather that's one of the main reasons why the visible spectrum of our eye is what it is (as opposed to the IR region or the likes)
     
  5. Jun 30, 2008 #4
    Isn't there also a reason related to the fact that a solid (at least crystalline solid) has energy bands while a gas is made of isolated molecules and therefore has energy lines? Could this mean that in white light a typical solid would absorb/block entire bands of wavelengths, while a gas would only block some monochromatic frequencies and therefore it would let most of the white light go through?

    ...or am I making this up? :)
     
  6. Jun 30, 2008 #5
    There are basically 2 reasons. For one, air is rarefied. The molecules are not very close together so that like waves (or particles) are not likely to collide with an air molecule along its trajectory. In other words, it has a very long mean free path.

    Also, and probably more important, none of the molecules that make up air absorb the frequencies of light that are visible to the human eye. Therefore, they do not absorb light and re-emit it at a different frequency as colored things do and that makes the air transparent to visible light.
     
  7. Jun 30, 2008 #6
    If your eye was built for a different wave-length, say at gamma ray level, you would be asking "why is aluminium transparent" instead.

    Basicly, the frequency of a wave decides how it interacts with different materials. The range which we call "visible light" happen to pass through glass unaltered, while the range we call heat-waves does not.

    This is one of the reasons why your car gets so damn hot in the summer: Light comes though the windows and bounces from the interior. It looses some energy during that bounce, and becomes a heatwave. Same thing, different frequency. Only, heat does not pass unaltered through the glass, so it is trapped inside and bounces back and forth, heating your car.

    k
     
  8. Jun 30, 2008 #7

    Andy Resnick

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    This is not true- simply allowing the IR radiation to get into the car and be absorbed by the car interior is sufficient to explain the behavior. One not need presume that visible light be 'down-shifted' to IR radiation, nor that IR radiation be 'captured' within the car interior. Neither occurs.

    Finally, a small quibble- heat and IR radiation are not the same things.
     
  9. Jun 30, 2008 #8
    kenewbie's response, also works with the visible light, but like Mr. Resnick said, there is no redshift of that light into IR. The materials in the car may absorb some of the visible light converting that energy into heat. But as Mr. Resnick said, heat is not light; thus is gets trapped in your motorized greenhouse.
     
  10. Jun 30, 2008 #9

    DaveC426913

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    I though the idea here was that windows are transparent to visible light but somewhat opaque to IR.
     
  11. Jul 2, 2008 #10
    So if a hight intensity laser beam was adjusted to one the frequencies of oxygen visible spectrum, would light scattering be observable ?
     
  12. Jul 2, 2008 #11

    Gokul43201

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    The best answer (in the sense that it provides the full overview) is the one given by Domenicaccio. Gases are transparent because their absorption spectrum is discrete rather than continuous. The reason for the discreteness is the inability to support collective excitations because of the weak interaction strength between molecules that are widely separated.
     
  13. Aug 19, 2009 #12
    None of these answers make sense. If we see though air (i.e., it appears transparent) because it's molecules are less dense than light waves of the visible spectrum, how do we "know" that the light comes from the surfaces of objects. That is, how do we attribute the light to the surface and not the air in front of the surface?
     
  14. Aug 19, 2009 #13
    None of these answers make sense. If we see though air (i.e., it appears transparent) because it's molecules are less dense than light waves of the visible spectrum, how do we "know" that the light comes from the surfaces of objects. That is, how do we attribute the light to the surface and not the air in front of the surface?
     
  15. Aug 19, 2009 #14

    DaveC426913

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  16. Aug 19, 2009 #15

    DaveC426913

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    What?10char
     
  17. Aug 19, 2009 #16

    sylas

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    That's true; but I think the more important factor for your car heating up is that the air gets trapped by the roof and sides. Outside, the air heated at the surface is continually replaced by cooler air from aloft, by circulation.

    The question of the thread can be rephrased: why are our eyes particularly sensitive to wavelengths where there is the most light passing easily through the air? The visible portion of the spectrum is both the peak of the solar radiation, and also in a band that does not interact with atmospheric gases.

    Cheers -- sylas
     
  18. Aug 20, 2009 #17
    The question is really straight-forward. Light waves reflect off of objects that then stream into the eye by first traveling from the object through open space. How is it that we determine where the object is located? That is, why don't we "see" the stream of light waves as they travel in the open space (so that it appears like a colored volume rather than a distant opaque surface)? Instead we somehow decide that the stream of light originates at the surface of the object and then we eliminate our perception of that stream from the intervening space.
     
  19. Aug 20, 2009 #18

    sylas

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    No, that's wrong. The reason we don't see the stuff in between is it has no effect on the light; it really DOES originate at the opaque surface. We don't see the light as coloured because it ISN'T coloured. If something is transparent... like the air... then light is passing through it without any effect. There's no special role for "eliminating perception". A machine measuring that light would also be measuring the surface that emits the light, just like the eye.

    Cheers -- sylas
     
  20. Aug 20, 2009 #19
    But the question remains, how does the eye (or a machine) "know" that the light originates at the opaque surface (and not closer to the eye)? Machines don't have depth perception. They only "see" in two-dimensions, without imposing some amount of empty (transparent) space between their receptors and the surface.
     
  21. Aug 20, 2009 #20

    sylas

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    You can have depth perception with machines just fine; it's not magic.

    Your question makes no sense. It's not about "knowing". The point is that the light ACTUALLY DOES originate at the surface -- whether you know this or not, and regardless of whether the light is received by a thinking being capable of "knowing" things, or a dumb machine that just makes measurements.

    That's why it is incorrect to say that there's some kind of special "perception" process involved in eliminating the effects of stuff in between. There's nothing to eliminate.

    There are certainly some odd things that can happen with perception. Here's my favourite; the human perception system sees blue and green spirals; but if you test the image more carefully, you can check that they are really the same color.
    [​IMG]
    (Reference: The blue and the green -- Bad Astronomy Blog).

    The point is, the invisibility of air is NOT a perception effect. It is because the air actually is transparent to visible light. This question is one of the physics of light and matter; not about perception and knowledge.

    sylas
     
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