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Light absorbance & color of materials

  1. Apr 8, 2013 #1
    it's known that for the visible part of EM wave(electromagnetic wave) , black materials absorb more visible light than white materials do . a friend of mine asked me if in general , black materials absorb more EM radiation(whose frequencies do not lie within the visible range) than white materials do . my 'conjecture' is that since the opacity of a material depends on its molecular structure(like the vibration/oscillation modes of CO2 cause the molecules to strongly absorb IR..?) , just because black materials can absorb more EM radiation than white materials do in the visible range doesn't directly mean that they can absorb more radiation of other frequencies , as those vibration modes could have no effect on its opacity to EM radiation other than visible light. As i'm not so sure about the concepts and the logic , I wonder if anyone could help me identify any falsehood/flaws in my claim..thanks in advance for paying attention to this thread..
     
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  3. Apr 8, 2013 #2

    mfb

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    They do not. Different frequency ranges are mainly influenced by different, independent phenomena.
     
  4. Apr 9, 2013 #3
    how so ?
     
  5. Apr 9, 2013 #4

    mfb

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    Gamma rays -> sensitive to nuclei, and total density of electrons
    X-rays -> sensitive to nuclei and inner electrons, and total density of electrons. In crystals, the lattice structure is relevant, too.
    UV and visible light -> chemical structure of the material, band structure, outer electrons
    infrared -> oscillations and vibrations of atoms and molecules, plus the same as UV/visible light depending on the material
    everything with longer wavelengths -> conductivity, possible polarization of the material

    That is just a rough categorization - it depends on the material, and most effects do not have sharp lines where they begin/end to occur.
     
  6. Apr 11, 2013 #5
    so is the fundamental factor the average range that charges can jump between energy levels?
     
  7. Apr 11, 2013 #6

    mfb

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    I don't understand that question.
    Fundamental factor of what?
    What do you mean with "average range"? Average over what?
    Charges do not jump between energy levels, systems can do that (if you do not count the photon as part of the system).

    The reaction of materials to light of different wavelengths mainly depends on reactions with energy differences similar to the photon energy, right.
     
  8. Apr 12, 2013 #7
    fundamental factor that determines any materials' absorbance to light ;
    i meant to say ' the average distance' ;
    i thought charges changes their distances with other charges as a result of conserving energy from absorbing light ( such that the energy of photons-> electrical potential energy of the charges) , but since these distances are discrete , so a certain material can only absorb photons of certain energy
     
  9. Apr 12, 2013 #8

    mfb

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    Many effects are relevant at the same time.
    That does not answer the question what "average" means.
    I don't think that is a useful model - this "distance" is not even well-defined.
    They are not. Energy levels are.
     
  10. Apr 12, 2013 #9
    here i just want to use a hydrogen atom to illustrate what i meant to say by those words.
    As far as i know electrons (not sure if they have to be in bound states) could have random positions . so , it seems that the electron has different positions at different time : the electron could be thousands of meters away from the nucleus and suddenly become extremely close to the nucleus..it's just that not all of these positions are equally probable . so i was thinking that there could be an average of all these "distances"...or is it better to use ' the most probable distance' at certain energy levels?(by 'distances' , i think i meant to say 'relative positions')

    By the way , are electronic energy levels related to the relative positions of charges (the principle quantum number increases as the electric potential energy in the orbital increases)? if yes , then why aren't these 'distances' discrete as well ?
     
    Last edited: Apr 12, 2013
  11. Apr 13, 2013 #10

    mfb

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    You cannot apply a classical view to the hydrogen atom. It just does not work. The electron does not have one position at a time*. You can calculate the expectation value of the distance, but that is not an average over time - it is an average over many measurements at many (different!) atoms.
    The (distance) expectation values of energy eigenstates are discrete. A particle does not have to be in an energy eigenstate, however.


    *in the de-Broglie-Bohm theory, it has, but even there you need the wavefunction (called "pilot wave" here) which is spread out. The particle does not influence the wavefunction (or anything else) at all.
     
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