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Why is glass transparent?

  1. Apr 27, 2013 #1
    I am not able to understand why is glass transparent because the same amount of atoms are still present and even if they are not able to arrange themselves in a proper crystalline structure then to there will some places where their would be less gap and some places where there is more gap for light to be able to pass through!
  2. jcsd
  3. Apr 27, 2013 #2


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    Hi RohitRmB! :smile:
    Light isn't bothered about where the gaps are.

    (after all, light slows down even when it does "go through the gaps")

    Light can be absorbed (and re-emitted in a different direction perhaps) even when it "goes through the gaps" if it has the right energy (ie the right wavelength) to be absorbed,

    but will pass straight through if it does not have the right energy to be absorbed …

    The electrons in different materials vary in the range of energy that they can absorb. Most glasses, for example, block ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum.​

    see http://en.wikipedia.org/wiki/Transparency_and_translucency for details :wink:
  4. Apr 27, 2013 #3

    I don't understand it either. As far as I can tell, there is no simple answer. You have to do some very complicated math and that's what drops out. Or so I gather.
  5. Apr 27, 2013 #4


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    No, it is not that hard to understand, at least on some conceptual level.

    In every solid electrons can occupy so called bands of different energy. There are two bands - one called valence band, occupied by electrons strongly attached to their atoms/molecules, and conduction band in which electron can freely move throughout the solid. Conduction band always contains electrons of a higher energy. (Compare http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html)

    Light doesn't get absorbed by solids just because, it gets absorbed when it can excite an electron from one band to another (the photon energy gets into exciting the electron). Whether light can get absorbed by a solid depends on band energy levels (or more precisely, on the energy difference between these levels). In some substances the distance between bands is such that visible light photons can't excite the electrons - they have too low energy. In effect visible light photons go through these substances without problem. Glass is an example of such a substance - which is why it is transparent.

    Also note that glass doesn't have to be transparent, as technically glass means any solid which is amorphous (doesn't have a crystalline structure) and undergoes glass transition when heated. What I wrote above explains why SOLIDS are transparent, regardless of whether they are glasses or not (think calcite crystals).
  6. Apr 27, 2013 #5

    OK. So what's the difference between glass and sand?
  7. Apr 27, 2013 #6


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    Sand is not a continuous solid, but every grain of sand separately will behave exactly the way I described. Depending on its exact composition and band energies it will either allow the light to pass through, or not.
  8. May 4, 2013 #7
    When the material is suddenly and rapidly cooled then the atoms inside doesn't get enough time to arrange itself but wherever and however they may be arranged, when light falls on it there would be some light that would get refracted, some would diffract, some photons which strike the nucleus would bounce back. so how come there is no loss when light passes through glass!!
    i am not able to imagine that how light will light actually travel.
  9. May 4, 2013 #8


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    You might want to start by reading our FAQ subforum, and in particular, this entry:

    https://www.physicsforums.com/showthread.php?t=511177 [Broken]

    It presents a naive view of optical transport in solids, but at the very least, it introduces the importance of the material itself, the collective phenomena that do not exist in isolated atoms, and the complexities of optical transport.

    Last edited by a moderator: May 6, 2017
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