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Absorbance spectrum of water

  1. Oct 8, 2015 #1
    Folks, I am looking for a more intuitive explanation of why water is transparent in the visible range. I am looking for the mechanism -- clearly it is transparent because photons are not absorbed. However, I am clueless as to why water should strongly absorb microwaves due to its polarity but fail to do the same for visible wavelengths. Can any one point me in the right direction, please? I have read that visible light is "too energetic", but water clearly absorbs some even higher frequencies, too.
     
  2. jcsd
  3. Oct 8, 2015 #2

    BvU

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    Did you check out this lemma ?
     
  4. Oct 8, 2015 #3
    I did. Am I missing some explanation from this page? I don't see any description answering my question unless I've overlooked it.
     
  5. Oct 8, 2015 #4

    BvU

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    There are no modes with energy level differences that can be excited by photons with wavelengths in the visible range. Shorter wavelengths can excite electronic transitions, longer wavelengths do vibration and rotation. These two energy regions don't overlap that much, so most of the visible range goes through fairly undisturbed. There's nothing to overlook.
     
  6. Oct 8, 2015 #5
    That's the root of my question. Why would visible light not also rotate water molecules, then?
     
  7. Oct 9, 2015 #6

    BvU

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    Let's ask @Orodruin :smile: because All I can think of is that the energy in such photons doesn't match a suitable energy difference in the spectrum of allowed rotation/vibration transitions. There will probably also be other conservation laws at work, not just energy (angular momentum, ..)
     
  8. Oct 9, 2015 #7

    Orodruin

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    Because it does not have the correct frequency to do so. The vibrational and rotational spectrum of water molecules lies in the IR to microwave part of the spectrum. In the other end, the UV part of the spectrum, other scattering processes take over and water becomes opaque for those frequencies too.
     
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