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UV light absorption by transparent sheets – how?

  1. Jun 11, 2012 #1
    Ok thanks for the responses to my other question; I think I now have it.
    I have a follow up question – that is almost on the same topic (but not quite)

    The complete spectrum (arund 200nm to 380nm) of UV light is absorbed by some plastics??

    Elections will be moved to a more energetic orbital when absorbing photons with the same frequency, any wavelengths with non similar frequency with be emitted (e.g. red flower) or transmitted (e.g. glass). Water doesn’t use this electron reflection/absorption as it “just happens” to have the same frequency in the hydrogen bond that will absorb a portion of the lower (red) end of the spectrum.

    This is all well and good UNTIL I got to thinking about something like a clear polycarbonate sheet.

    If absorption is due to specific frequency of electron vibration – how can a polycarbonate sheet (or most types of glass and lots of other “stuff” for the matter) absorb virtually all wavelengths between around 200nm to 380nm. (UV a,b and c)

    Surely there must be something else going on.

    How does a polycarbonate sheet (e.g. sunglass lens) absorb ALL ultraviolet light. It must be a different process as I don’t think there that many electron frequencies. I have looked on google for many days and there are many descriptions of the results but I can't find one that gives a complete (physics) reason.

    Thank you


    LoL – this all follows on a from a discussion my family were having regarding the colour of beer and as I kept digging deeper and deeper more question seem to appear. I hope you all don’t mind me asking such (what must be to you) simple questions.
     
  2. jcsd
  3. Jun 11, 2012 #2

    Simon Bridge

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    Why do you believe it has to be a different mechanism?
    What's wrong with the band structure of a polycarbonate sheet having resonances for UV?
     
  4. Jun 11, 2012 #3
    Well I “assume” that there is a similar process taking place for most reasonably similar wavelengths.

    May be totally wrong about that.

    Say you have a blue ball. It has absorbed all wavelengths other than (for example) 470nm as there are no electron frequencies of 6.3786e+14 (well that’s what my calculator says it should be :-). For a polycarbonate sheet to use the same process it would mean that there were "no" (not one) electrons with frequencies of between 1.4990e+15 and 7.8893e+14 (200nm to 380nm) - I have not included VUV in this.

    Now I may not have a big enough calculator (and I am not a statistician) but let’s just say that between 1.4990e+15 and 7.8893e+14 there are a “lot” of possible frequencies. Surely there would be a least a few electrons with the same – there must be?. Would this not lead to some transmittance. But it is claimed that aren’t.
     
  5. Jun 11, 2012 #4
  6. Jun 11, 2012 #5

    Simon Bridge

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    Hmmm OK.
    In contrast - the actual spectra is more complicated than they show. eg.
    media.nl?id=6279&c=621686&h=995dc7615bb5b6774b1e.gif
    ... see how the transmission bottoms out at short wavelengths? Remember that short wavelengths = higher energy.

    Above a certain energy the energy band is basically a continuum in solids so you get a whole chunk being absorbed like that.

    Get to a higher energy and different processes start to dominate - at very high energy you have matter and that just punches holes in things.
     
  7. Jun 11, 2012 #6

    sophiecentaur

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    This is a common misconception about what happens in condensed media (solids, for instance). It is much too simplistic a picture of what goes on to talk of orbitals and levels. In solids, there is not a set of quantised energy levels but a whole band of energies that electrons occupy. All the atoms in the vicinity of a given atom contribute to the energy of the outer electrons.
    Colour due to absorption in pigments tends to be very broad band and not like the pictures of absorption lines in gases (which you see in textbooks).
     
  8. Jun 11, 2012 #7
    Thanks for that.

    I need to do some more reading :redface:
     
  9. Jun 11, 2012 #8
    I have found this on wiki but (as you can see) it does not go down far enough.

    http://en.wikipedia.org/wiki/File:Polycarbonate_IR_transmission.png

    Does anyone have a link to a (peer reviewed) paper with the same info but going down into the UV area
    I say peer reviewed to keep away from the vast amount of rubbish put out by sun glass makers ?

    Thank you
     
  10. Jun 11, 2012 #9

    sophiecentaur

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    Don't worry. We all need to do more reading on lots of stuff. There's no end to it. ;-)
     
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