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What's so special?

  1. Jul 28, 2005 #1
    Hey there!

    We know that,
    1. No light passes through an opaque objects.

    2. Light passes partially through translucent objects.

    3. light passes completely through transparent objects.

    Now, the question is,
    What's so special about transparent objects that light passes through them and not the opaque objects?
  2. jcsd
  3. Jul 28, 2005 #2


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    Ah, but here's what you are missing.

    1. No light passes throught an opague object is partially right IF you are restricting yourself to the visible range of the EM spectrum (which is what most people do since "light" is typically defined as the visible spectrum). However, you will note that x-ray can pass through even a thick piece of object. In fact, there are a large range of EM spectrum that will pass through objects that we characterize as "opaque".

    2. skip

    3. light passes completely through transparent objects... or do they? Take a piece of ordinary glass, and while it is transparent for visible light, it is a very effective blocker of UV radiation and a range of EM radiation. So glass is actually opaque to some range of "light".

    What's the point here? The point here is not that there's anything "special" about the material, but rather you should question on what's so special about the VISIBLE range of light in the first place. Almost EVERY object has a range of EM radiation in which it is opaque to, and another range in which it is transparent to (for a suitably-given thickness). We just happen to be able to detect a very small range of the EM radiation. But there's nothing special about that anymore than there's something special about gamma radiation.

    I'd rather not go into the mechanism of optical conductivity because, heaven knows, I've gone through that, it seems, forever. You may want to do a search on PF for that.

  4. Jul 28, 2005 #3

    What's special about a transparent object is that the surface is smooth
    compared to the wavelengths of visible light, roughly about 0.00055 mm.

    If the surface gets rough (like if you scratch up a window with sandpaper)
    then the little scratches prevent the light from going through strait. They
    still get through, but they get scrambled and this appears as translucence.
  5. Jul 28, 2005 #4
    That's an incomplete answer though. I can polish steel to be smoother than your window glass and it won't be transparent.
  6. Jul 28, 2005 #5
    Yes. If the material is transparent then what I said applies. If it is not,
    then there are other reasons for that.

    Some atoms & molecules don't couple strongly to the energy as it
    passes by. If thats the case then it will be a transparent material.

    In other materials the atoms or molecules are strongly affected by the
    passage of the light energy. These materials will absorb the light and
    get warmed up by it.

    As ZapperZ noted, this depends on the material and the color of the
    light so yellow glass absorbs blue but passes green and red.
    Last edited: Jul 28, 2005
  7. Jul 28, 2005 #6


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    Locrian is correct. The smoothness, at best, is an incomplete explanation for optical transmission. The ONLY reason the smoothness would matter is because of the ANGLE of incidence onto the surface of the material. A rough surface would present a large range of angle of incidence even if that light has a normal incidence to the macroscopic surface. But once its in, it no longer matters and the dominant mechansm of optical transmission takes over.

    It is why people who do work in optical reflectivity and transmission (FTIR, Raman, etc) polish the surface that they're working on very thoroughly until they are as optically flat as possible. This is so that they can be sure that the photons they are using hit the surface at a uniform angle all over the surface. However, the rest of the optical transport properties have no bearing on surface roughness.

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