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Why aren't most insulators transparent just like glass?

  1. Apr 8, 2014 #1
    Hello Guys!

    I have been studying electromagnetic waves (EMW) interaction with matter lately and I just derived the results for the propagation of EMW inside linear media and perfect conductors. As it turns out, when a plane EMW changes medium (at a normal incidence) from air to (good) electrical insulators most of the EMW is transmitted (thus glass is transparent) and when going from air to (good) conductors most of it is reflected (thus metals are opaque and when polished are good mirrors). So I cannot help but wonder why aren't more materials transparent? Wood, paper, all these things that are good insulators are opaque as well.

    Well, I know that this is an incomplete picture since it considers the EMW as plane and homogenous and does not take into account microstructure and quantum effects that arise from more accurate matter description. I believe one of the main processes that render opaqueness to most insulator materials is due to non-normal incidence of light into granulated matter that scatters off in all kinds of complicated ways that make them opaque. For example, ice is transparent when it is fabricated in such way as it is nearly a monocrystal but it looks white when it has a granulated (policrystalline) structre (walls of your freezer). If we could fabricate insulators such as rubber or most ceramics in a homogenous way would they be transparent to visible light?

    I believe the the usual lack of homogeneity and granularity of most materials make them opaque but common glass or quartz (SiO2) is amorphous and is still transparent...so why aren't more insulators transparent?

    Thanks for reading... sorry if this post sounds confusing...it was written by a confused mind hehe.
  2. jcsd
  3. Apr 8, 2014 #2
    The emr interaction with matter can only be studied with quantum mechanics. The classical theories are either based on the quantum mechanics, or are empirical (or just plain wrong).
    What is this granulated matter of yours? Scattering due to a change in refractive index is a very important source of opacity. The larger the difference in Nd, the greater the scattering.
    Isn't lack of "homogeneity" implicitly a change in Nd? You use the terms in a way that makes me question your knowledge of chemistry. There is nothing magical about visible light, except we evolved to see it. There ARE reasons, chemical (as well as physical), that our eyes are sensitive to the frequencies which 'happen' to have so much interaction with matter (how useful would gamma ray eyes or microwave eyes be, compared to ours?).
    Many silicone rubbers are clear. If you are among the majority of first worlders and wear glasses, chances are your lenses are plastic and amorphous, not crystalline. You have the theory (wrong, since it is (I assume) classical theory) about how a perfect conductor is opaque. You really should be trying to understand why something that supposedly conducts so well doesn't allow emr to pass. It seems to me that THAT should be what bothers you, not that things which don't allow emr to pass also make it difficult for electrons to move.
  4. Apr 9, 2014 #3


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    If you take any transparent material and crush it to a fine powder (glass, sugar, salt) you will find that the powder is white - this is due to scattering from all of the small surfaces.

    Some insulators have wide absorption bands; wood is such a material. Break it up into a fine powder (saw dust) and it looks the same as before. Some materials have a lot of internal scattering - they may appear translucent, such as some types of plastic and glass. Or there may be so much scattering that you cannot see through them at all.

    The typical transparent material has limited scattering and absorption; it also lacks the free electrons which make metals into conductors.

    All of this theory is studied in condensed matter courses, usually after you have taken quantum mechanics and statistical mechanics. The results of electromagnetic field theory are quite limited as it does not take into account the actual properties of materials. Thus you have index of refraction (a measured quantity, not calculated from theory) ... you need a deeper theory to calculate it.
  5. Apr 9, 2014 #4
    You derived your result for a specific frequency band?
    These results are not general. For example glass (an insulator) is not transparent in UV but some metals are.
    However many insulators are transparent in visible spectrum, you are right. If some plastics, waxes, paper, etc do not look "transparent" is due to what was already explained.
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