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Why is Glass Opaque to IR and UV, But Not Visible Light?

  1. Oct 2, 2011 #1
    According to my understand of what makes something transparent, it's because a given wavelength of light doesn't have sufficient energy to raise the electrons in a material to a higher energy level, so the photons are not absorbed and continue on.

    This explains why glass is transparent for visible light but absorbs UV, as UV has enough energy to excite the electrons to the allowed energy levels of silicon...but then why is glass also opaque to lower energy IR?

    Thank you.
     
  2. jcsd
  3. Oct 2, 2011 #2

    phinds

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    Hm ... if glass is opaque to UV and IR, I wonder why it is that my car seat gets so damned HOT in the summer?
     
  4. Oct 2, 2011 #3

    NascentOxygen

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    Don't solar cells (silicon) use mostly visible light? So there must be a fair amount of energy in the visible spectrum of sunlight.
     
  5. Oct 3, 2011 #4
    It's actually because glass is transparent to visible light, which is absorbed by the materials inside your car and then that light is re-radiated at a longer wavelength--IR. This IR is then trapped inside of the car because the glass windows keep it in.
     
  6. Oct 3, 2011 #5

    davenn

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    ahhh but the flaw in your logic is that the glass window let the IR in for a start

    Glass doesnt stop UV or IR ... if it did IR or UV cameras would not work they ALL have glass lenses
    the only time glass WONT pass either IR or UV is if it is specially treated to produce a filter to those wavelengths. for example I have a screw on UV filter on my digital SLR camera
    I wouldnt need that if glass normally stopped UV

    cheers
    Dave
     
    Last edited: Oct 3, 2011
  7. Oct 3, 2011 #6

    davenn

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    actually looking at the first part of your post, that isnt quite right. According to the late Prof. Richard Feynman, basically, photons entering the glass are absorbed by the atoms of the glass exciting electrons causing those electrons to emit new photons that carry on through the glass.
    that is, I understood from his lectures, that the photons coming out the other side of the glass (or those that appear to be reflected out the same side) are not necessarily the same ones that went into the glass.

    cheers
    Dave
     
    Last edited: Oct 3, 2011
  8. Oct 3, 2011 #7

    davenn

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    Another flaw in your logic ...... ;)

    in a ventilated area, put your arm under a sheet of glass in summer sun, I bet it gets seriously sunburnt. The IR energy isnt getting trapped anywhere except in your skin!! :)

    Not sure where your got your understandings from, but unfortunately, you were seriously misled

    cheers
    Dave
     
  9. Oct 3, 2011 #8

    NascentOxygen

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    He didn't say that it does. In fact, he states that it doesn't.

    Wrong.
     
  10. Oct 3, 2011 #9

    NascentOxygen

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    IR? Sunburn? Really?

    Another flaw in your logic ...... ;)

    Not sure where your got your understandings of sunburn from, but unfortunately, you were seriously misled
     
  11. Oct 3, 2011 #10

    davenn

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    yes he does read his original comments!! he states IR and UV doesnt pass through glass


    try again as I said if glass stopped UV then there would be no need for me to use UV filters!!!

    I really dont know if you are just picking a fight/argument
    or you really have no understanding of optics

    I suspect the former and you are just wanting to be nasty
    in which case I will leave you to your stupidity as I have no inclination of trying to deal with the likes of you any further
     
  12. Oct 3, 2011 #11
    wiki:
    http://en.wikipedia.org/wiki/Ultraviolet
    UV is from 10 to 400 nm, so no, you won't get a sunburn trough the glass.

    http://en.wikipedia.org/wiki/Greenhouse
     
  13. Oct 3, 2011 #12

    NascentOxygen

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    That's what I said he said.

    If glass didn't heavily attenuate UV then there would be no need for vets to warn pet owners that sitting bird and reptile cages beside a window will not provide their pets with adequate daily UV light unless the the window is actually open.

    Just gently correcting your misunderstanding, actually. I think where you're going wrong is with your perception of attenuation as an "all or nothing" thing. It isn't. While all frequencies of light are transmitted through ordinary glass, UV is more heavily attenuated than is visible light.

    <rant elided>
     
  14. Oct 3, 2011 #13

    sophiecentaur

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    I don't think that can be quite right - at least not when it's put in that blunt way. If new photons were generated as a result of old ones being absorbed, the re-radiated em waves would be in all directions so no coherent image could be seen. ( You couldn't identify actual 'Rays' going through.) The only time that photons are generated in this, coherent, way is during Laser operation and that only happens when Stimulated Emission takes place.

    It is true to say, however, that photons do react with the medium they pass through, though (the velocity of the wave slows down) so there is 'something' in what you say - it's not just that simple, I think.

    Transmission of em through materials is different for all frequencies (energies). Gamma rays tend to get through any but the most dense metals fairly easily - they just hit the nuclei and may get absorbed. Low energy (RF) waves do not 'pass through' metals but generate currents by generally 'vibrating' all the free electrons in the metal (in a 'bulk' interaction - rather than a one photon / one electron interaction. This causes reflection plus some absorption.

    For crystals and amorphous (covalent?) materials, (insulators, at least), there can be slight shifting (polarising) of the charges within molecules which may involve more or less loss / absorption, depending on the wavelength and the material. Perhaps this is somewhere near what you got from the Feynman lecture? He did love his Feynman Diagrams but he stressed that they are only meant to be a short hand version of what goes on and shouldn't be regarded as an rigorous description of a 'mechanism'.
     
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