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Absorption (without energy level transition).

  1. Jul 8, 2010 #1
    This thread is related to another question that I have been pondering.

    In most objects that we experience on a daily basis (a green shirt for example), we know that most of the frequencies of light striking the object are absorbed.

    I simply can't believe that the atoms in the shirt have enough possible electron energy levels that would allow for so many frequencies to be absorbed. (plus they would just be re-emitted when the electrons decay back to ground state, with no net effect).

    Does this mean that there is absorbtion that takes place that simply shakes electrons without making them jump from one energy level to the next, thus absorbing light without re-emittting.

    Can this actually happen?

    Once again, thanks in advance for any help in the matter.
     
  2. jcsd
  3. Jul 9, 2010 #2
    are they? this assumption is the source of your confusion. Atoms only absorb certain energies, not the whole electromagnetic spectrum.
     
  4. Jul 9, 2010 #3
    I have a more fundamental question on similar lines: we know that in an atom, only certain discrete energy levels are available for the electron in it to transition. Then consider the case, when a photon of an energy that is mid way between two consecutive energy levels, and the electron (for simplicity, let us assume Hydrogen atom) is in the lower energy level. Now, the photon does hit the electron, but since its energy is not adequate to cause the electron to jump to higher energy level, what then actually happens? Does the photon simply pass through the atom, without interacting with the electron, as it doesn't have the required amount of energy to excite it, or is it just absorbed in the electron, and the electron recoils with an equivalent amount of energy? The second condition doesn't seem to be correct, as the recoil energy should also be in accordance with the discrete energy levels allowed for the electron, but I need opinions of others on this. Another possibility that comes to mind is that photon creates the electron-positron pair when it is near the atom.
     
  5. Jul 9, 2010 #4

    alxm

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    I assume you mean most visible frequencies? Obviously most of the electromagnetic spectrum is not absorbed (microwaves, radio waves, gamma rays, etc)

    The problem is that you're thinking in terms of atomic absorption spectra, for something which is very far removed from being a bunch of individual atoms in a vacuum.
    What's the difference between the spectra of an atom and a molecule?
    What happens to the spectrum of a gas when you increase the pressure?
    What happens to the spectrum when it becomes a liquid, and a solid?

    Figure out the answers to the above and you'll know why, any introductory book on solid state physics or physical chemistry will tell you.
     
  6. Jul 9, 2010 #5

    alxm

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    By order of probability, either:
    1) It passes through and nothing happens
    2) Rayleigh (elastic) scattering occurs, the photon and atom change direction due to the interaction of its electrical field with the electron(s). No transfer of energy occurs, and the electrons do not change state.
    3) Raman (inelastic) scattering occurs, the photon transfers some of its energy to an electron or vice-versa, leading the electron to transition to a higher or lower energy state, and the scattered photon will have a lower or higher frequency correspondingly. This can also be viewed as absorption to a fictional "virtual" level, followed by immediate re-emission back to a different real level.
     
  7. Jul 9, 2010 #6

    ZapperZ

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    You may want to start by reading the FAQ thread in the General Physics forum, especially on the post dealing with photons moving through a material. In it, there is an important distinction made between the study of atomic physics, versus solid state physics, in which many atoms and molecules have come together to form a collective state.

    Important moral of the story: when a collective state forms, there are many properties that are not present in isolated, individual atoms/molecules. A "conductor" or "insulator" is an example of a collective phenomenon whereby band of states are formed that are not present in individual atoms. In this case, the collective phenomenon that is relevant in light absorption and transmission are the collective vibrational modes, or phonons.

    Zz.
     
  8. Jul 9, 2010 #7
    Zapper - Thank you very much for the clarification. It turns out that I originally thought that was exactly the explanation of light absorbtion in solid state materials by figuring it out on my own. It seems very analogous to things that I have observered in working with acoustics in my vehicle.

    However, I was lured into many of the quantum mechanics discussions, and it would lead you to believe that photons and energy states are the only things in the world that matter.

    Thank you for the clarity. It is much appreciated.
     
  9. Jul 13, 2010 #8
    I know a photon has an electric and magnetic component . But why would a photon change direction due to an E field. I thought if i shoot a photon trough an E or B field , it would not deflect .
     
  10. Jul 13, 2010 #9

    alxm

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    The photon's E field interacts with the charged electrons, inducing a dipole moment which modifies the incoming photon field. It's fairly straightforward classical electrodynamics, all one needs is to assume the scatterer is small relative the wavelength of the incoming radiation, and that it's polarizable. (See textbooks for the proper derivation of Rayleigh's formula)

    It's basically elastic scattering, like off a rubber ball, where the electrical polarizability is the elasticity of the material.
     
  11. Jul 14, 2010 #10
    Thanks for your answer alxm , So when the photo's E field interacts with the electron it induces a dipole moment in the photon or the electron , why cant i alter a photons field with a bar magnet ,
     
  12. Jul 14, 2010 #11

    alxm

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    Well, that's a magnetic field, but nevertheless it's a valid question.
    The answer's hidden in what I already said, actually: The scattering object must be small relative the wavelength of the photon. Figure out why and you'll have the answer.
     
  13. Jul 14, 2010 #12
    Is it related to the averaging effects of the field over the larger dimensions of the magnet, that we do not observe any effect on a photon due to such a magnet?
     
  14. Jul 17, 2010 #13
    After looking through the FAQ and doing other reading, I am now comfortable asking the question in a more specific way.

    What is actually happening when a solid object absorbs the full spectrum of visible light, minus one color that reflects?

    Do the electrons shake in their energy level without jumping?
    If so, does the whole atom that it currently belongs to accelerate with it?
    It the electron simply floating, like in a metal, and shakes on its own without being coupled to an atom?

    OR, is it required that an electron changes energy level in order to absorb light?
    This seems wrong to me, because those electrons would just end up decaying back and re-emitting that same photon. No net effect other than a scattering. This explanation does not seem to agree with observation.

    OR, maybe the electrons are just loose? But metals have this quality, and they reflect all visible light.

    It seems as if the first explanation is the most correct in this case. I really would like to resolve this.
     
  15. Jul 18, 2010 #14

    ZapperZ

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    Unfortunately, based on this post, I don't think you've understood what you read in the FAQ.

    The major "moral of the story" in the relevant FAQ thread is that when atoms conglomerate to form a solid, the collective behavior of those atoms now produce a new system in which the individual properties of the atoms are less significant than these collective properties. The vibrational modes that was discussed (phonons) simply do NOT exist at the individual atom's picture! The ability of a SOLID (not atom) to either reflect, absorb, or transmit EM radiation depends very much on these collective properties.

    Zz.
     
  16. Jul 18, 2010 #15
    I understand the idea of a lattice system in which the vibrational modes are a function of the collective interactions of the array of particles within this system.

    What I am asking is this:

    Does the absorbtion of visible light ever happen without an electron moving to another energy level. Can light just shake some electrons that are coupled to atoms and the net result is the atoms themselves shake without any electrons actually relocating to a new orbital.

    I guess I need a bit more information into how these vibrational modes typically behave.
     
    Last edited: Jul 18, 2010
  17. Jul 19, 2010 #16
    Any advice on places to look for material regarding the question - whether on not light is absorbed by electrons without an change in orbital.
     
  18. Jul 19, 2010 #17

    alxm

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    No, light cannot be absorbed by electrons without a change in orbital, by definition.
    Absorbing light would mean changing/increasing the energy of the electron, and since orbitals are the energetic states of electrons - it's impossible.
     
  19. Jul 19, 2010 #18
    In this case, it seems that no net light absorbtion would take place, as light would just be re-emitted at the transition back.

    This just does not appear to be in agreement with observation.

    So we have:
    1. axlm saying that energy level transitions must occur.

    2. ZapperZ saying that there is a "collective" circumstance, which I believe to be true, but he hasn't given me a clear explanation as to the real mechanics of what the light actually does when it gets absorbed.

    3. And me, thinking that light can effectively push on molecules/groups of molecules as a whole(because molecules are coupled to their electrons which are pushed by the light.)

    Which would be the closest approximation to reality?

    I apologize for my persistence with this topic, but I see it as an important basic understanding of how things work - something even a layperson should at least gather about nature.
     
  20. Jul 19, 2010 #19

    Cthugha

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    1. and 2. do not contradict. The collective system causes the energy levels in question to be energy bands with a continuous range of energies (instead of discrete energy levels for bare atoms). Absorption of light causes an electron to be excited from on of these bands to the other - for example from the valence to the conduction band.

    The third possibility is not happening. Exciting electrons to a higher state does not mean that they are shifted to another spatial position.

    But the light will not necessarily be emitted in the same direction the absorbed light was heading. The reemitted light will be emitted into all directions with equal probability, causing a much lower intensity of light of an absorbed color in that particular direction where the remaining light is going to.
     
  21. Jul 19, 2010 #20
    If this is true, then how is it possible to have a black object? One that re-emits NO light in ANY direction. It would seem that there would be a situation where electrons would become excited and staying that way (since no re-emission takes place). This would continue until all electrons are in that state, with no more available to do so, and the object would then turn white. It just doesn't seem to work.

    Now, why is option 3 "not happening".
     
    Last edited: Jul 19, 2010
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