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H2O Vibrational Transitions (molecular as opposed to election) Why ?

  1. Jun 1, 2012 #1
    Good morning.

    Yet another question ;-)

    I have been looking at so many websites and I still can’t find the answer, so if anyone could give me a simple explanation, or give a link to a web page, it would be really appreciated.

    Visible wavelength selective absorption/reflection.

    Water is blue as the H2 selectively absorb/emit various frequencies at the lower end of the visible spectrum. But in everything else is at a MUCH smaller scale (of the electron).

    I understand how electron frequencies selectively absorb / emit frequencies. But why is H2O different.

    I have found many explanations of the results but none that explains the causes.

    Why would virtually all other object’s electrons react to energy waves but the scale is much larger in water (where the hydrogen bonds contract, shake, expand, warp, etc) and make the H the “active” element in the absorption/emitting. (sorry can’t think of a better word than Active).

    I would have thought that if other objects containing either H or O have the H or O’s electrons selectively absorb the energy then the same should be true of water. But it isn’t????


    Is it the strength/relative length/configuration/etc of the hydrogen bond or something completely different?

    Sorry to ask so many questions but I really am trying to understand what is the difference between water’s "colour" and virtually everything’s.

    Thank you
  2. jcsd
  3. Jun 1, 2012 #2


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    It's the energy levels and gaps between them that determine the frequency of EM that is absorbed / emitted. Although we start off by looking at the Hydrogen atom in terms of the electron energy, it's really the energy of the proton-electron pair (both charges). In the case of molecules, there are charges distributed within the molecule and it is the energy associated with re-arranging these charges that determines the frequencies of EM that will interact with them. But, in a condensed medium like liquid water, all the electrons and protons in each molecule are influenced by the charges in the nearby molecules too. This results in interaction with spectral bands rather than with just lines. In the case of water, the longer wavelengths happen to be absorbed more than the short wavelengths so the light with a long path through pure water will look blueish (i.e. white minus the redish wavelengths)
  4. Jun 1, 2012 #3


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    The energy levels of the water molecules electrons only absorb radiation in the UV range. The visible light absorption is mostly due to the vibrational modes between the 3 atoms.

    Other materials do not have the same configuration as water, so their electrons may absorb energy in the visible range or not depending on the specific material.
  5. Jun 1, 2012 #4


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    Just referring to electrons is to miss out the important energy levels associated with the positive charges, too. That wiki link of yours, about vibrational modes, makes it clear that it is the nuclei as well as the electrons that are involved.
  6. Jun 1, 2012 #5
    Than you all for your time and knowledge.

    I "think" I have it now.

    Due to "vibration" of the H2O being at the same frequency of some of the spectrum (many the shorter wavelengths - greens, yellows, reds) it is those that are absorebed.

    But I stil don't understand why this would not be the case for anything with molecular movment - provideing it was within the frequency of any light hitting it.
    What makes water different?

    I understand that there will be multiple bonds - but isn't this the case with many "things"
  7. Jun 1, 2012 #6


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    Yebbut the devil is in the detail. It's the actual energy level values that count and hydrogen bonds in water are 'just right'. Nothing wrong with coincidences, you know! There will be other transparent substances that 'just happen' to absorb Infra Red, no doubt.
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