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Why doesn't air form shadows?

  1. Jul 13, 2014 #1
    I am guessing this is due to diffraction due to the small size of the molecules of air. But then, we never see diffraction effects (interference) for any arrangement or configuration of molecules in space?
     
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  3. Jul 14, 2014 #2

    CWatters

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    Can you expand the question a bit?

    A shadow normally forms when an opaque object block light. Air isn't opaque so why does it need a mechanism like diffraction to stop it making shadows?
     
  4. Jul 15, 2014 #3
    Just like the post above, but I'd like to add a few more details.

    Have you ever looked at your shadow when it bright and sunny? Move your hand further away from the ground... what will happen is that the shadow will look blurry. Why? As the above stated, diffraction comes into effect BUT more importantly, the smaller the object, the smaller the shadow. The smaller the shadow, the less diffraction is needed to hide or remove the shadow.

    So, simply, in a pinch, you could say that Air molecules are too small and light doesn't tend to hit them.

    Also note that light reflects off of objects too! So that means that light hits a single spot from multiple angles.
    That is why you can the the ground even though there is a shadow.
     
  5. Jul 15, 2014 #4

    Drakkith

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    I think this may have more to do with the sun not being a point-like light source, but an extended one. At the edge of the shadow you will have some of the ground that is blocked from the entire light source, but as you keep going, more of the light source is visible to the ground, leading to a gradual fading instead of a sharp edge.
     
  6. Jul 15, 2014 #5
    shadow to human eye implies absence of light, why would air cast shadow when most of the light passes through it.
     
  7. Jul 15, 2014 #6

    Chronos

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    A mirage is an example of air casting 'shadows'. Changes in air density causes both diffraction and refraction. The transmissibility of light through air is affected both by temperature gradients and turbulence.
     
  8. Jul 15, 2014 #7

    Drakkith

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    In this case the air is acting like a lens, focusing and redirecting light to different places than it would otherwise land. I personally wouldn't put this in the same category as casting a shadow, but that's just me. I suppose you can if you want to.
     
  9. Jul 15, 2014 #8

    sophiecentaur

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    The vast clouds of gas, out there in space, do actually 'cast shadows' when they lie between a source and an observer. By scattering light, they will reduce the intensity of the light travelling through them.

    This would happen whether light consisted of waves or corpuscles.
    It's a matter of geometry, not diffraction (remember the School diagram with Umbra and Penumbra?)
     
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