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Light moving through objects

  1. Feb 1, 2009 #1
    Why does radiation of the visible spectrum move through silicon, water, ice, glass and so on?
    I've been wondering about this for a long time, and have been looking at various texts on the net, and the most common answer found is that it a) is because the atoms in the compound are unordered, and/or b) because the electrons in for example glass transmit the photons passing through it.

    Now, none of the texts give a very good explanation to as what factor(s) are important when determining whether a compound will be transparent to a certain wavelength. As far as I've understood, the wavelength of the radiation passing through has a lot to say. Visible light goes through glass, but UV and IR do so poorly. Why? Is it because of a relation between the density of glass and the wavelength of light? Is it the number of valence electrons in each atom? Or is it the type of bonds between the atoms? Or even between the molecules?

    For instance, I can't find anywhere whether UV goes through silicon. Or better, whether x- or gamma-rays would go through a solid iron door.

    I've found this here at physicsforums, but that discussion doesn't give the fundamental reasons for whether radiation would go through something.

    Any comments at all are appreciated!
  2. jcsd
  3. Feb 1, 2009 #2


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    Gamma rays:

    Gamma rays go through anything, subject to an exponential attenuation, depending on the material and the gamma ray energy. The material property is a function of atomic number.
  4. Feb 1, 2009 #3
    Your question is not entirely well worded. All EM radiation moves through all media, however it is often absorbed very quickly, making them 'opaque'. As to why visible light is not absorbed by glass it is to do with the structure of the material.

    Unfortunately I think I'm right in saying that, from first principles, it is very difficult to calculate how much a solid will absorb radiation of a given frequency (it involves many variables). So a quantitative model is not really possible.

    Qualitatively, for visible frequencies it's the electrons that are really the important things. However the arrangement of the electrons obviously depends on the crystal lattice, the bonding, the atoms involved etc.

    For visible frequencies keywords like 'complex refractive index' and 'skin depth' might help if you're looking for material properties at specfic wavelengths.
  5. Feb 1, 2009 #4
    skin depth,,humm ..did i not hear a urban thingie that GI's marched to watch a nuc blast could see the bones of their hands held up to block the intense light?
  6. Feb 2, 2009 #5


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    Please start by checking an entry in our FAQ thread in the General Physics forum.

  7. Feb 2, 2009 #6
    I'm not sure what that has to do with the topic, the 'skin' in the 'skin depth' I'm referring to is nothing biological.

    However, yes it's probably true. If you get a bright torch on the other side of your hand a look at it in the dark then light will get through...
  8. Feb 2, 2009 #7
    i understood that ... was jest referring to that depending on the energy level and freq..
    and the materal pentration of light is not the same
    i was pokeing on that one..

    water is accepted as transparent by most
    but enough and it is blue freq that shows and more yet it is black..

    photons are not x rays and the org question did not address thickness of the materal nor engery level and to ask if gama woud go through iron?

    x rays will, depending on the engery emmited and thickness of the iron door
    water is verry "transperant" to x rays unless it contains boran and light will pass through boran easly..
    so while i dont know the "physics oe math" of it i "think" it is a case of apples and oranges and tomatoes
    while all food they are not the same at all
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