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Lights interaction with matter

  1. Sep 15, 2003 #1
    [SOLVED] Lights interaction with matter

    When light (photons/waves/whatever) comes into contact with matter, what are all the possible outcomes? Anyone mind listing them out with an explanation for each?
  2. jcsd
  3. Sep 16, 2003 #2

    Chi Meson

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    This is a partial list with much detail (deliberately) left out.

    1. The photon could be absorbed. The enrgy of the photon ends up as molecular motional energy (thermal) and temperature goes up.

    2. The photon could be re-emmitted. If the energy of the photon does not match an electron energy-level jump, the photon will "come back out." THis is what happens when light goes through transparent material. THe absorption/re-emmission is what "slows" the progress o fthe light through the material.

    3. The photon could be scattered. Sortof like the last one, but the re-emission is in a random direction, rather than the same direction.

    4. Stimulated emission: if a photon hits an energized atom, where an electron is at a higher energy state, and if the energy of the photon is the same as the energy-level "jump" of the electron,then two idential photons will be emitted.

    5. Flouresence: an atom absorbs a high-energy photon, but re-emmits several lower energy photons, where the total energy of the emmitted photons equals the energy of the absorbed photon.

    um...there's more
  4. Sep 16, 2003 #3
    Thanks for the response!

    Few questions...
    What's required for the photon to be absorbed w/o re-emission and go twords raising temp?
    The last 4 basically all involve absorption and then some kind of re-emission. What controls the type of emission that takes place?
  5. Sep 17, 2003 #4

    Chi Meson

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    Two Words:

    quantum probabilities.

    A lot has to do with the electron energy-levels. If an atom or molecule has an allowed energy transition (means: "size of jump for electron")that is equal to the energy of the photon, then the electron can stay at that level for a long time(minutes) or a little time (nanoseconds) or something in between.

    THe atom/molecule can be set into vibration, rotation, or linear motion which is manifested as "molecular motion" which is what thermal energy is. There are quantum rules that I have forgotten as well as those I never knew that allow us to determine the probability of each the various outcomes.

    Infra-red light just happens to have the right energy level to cause lots of molecules to "resonate" because they have electron energy transitions that match. Therfore, IR radiation is felt as "heat."

    Ultraviolet light is higher energy and there are lots of compounds that will absorb UV because the photon energy matches a large "multiple jump" for an electron, but when it descends, it takes several lower energy jumps, each emmitting lower energy light.

    I still can't expand that list, but I know I'm leaving something out.
  6. Sep 17, 2003 #5

    Chi Meson

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    Oh yeah. Duh. reflection. When a photon hits a metal surface it encounters "free-electrons" which are not behaving the same as electrons that are stuck in energy-levels. THe difference between reflection and absorbtion/re-emmission or scattering, is that there is no polarization caused by metallic reflection.

    And as I recall, those quantum rules are a bunch of mathematical wave formulas for which there are not many decent analogies that allow a mental picture of "why" these things happen.
  7. Sep 17, 2003 #6
    Chi Meson,

    I got chewed out a few weeks ago
    for saying that electrons changing
    orbit could radiate in the infra-
    red range.Do you have any links
    to a site that would constitute
    an authoratative statement about
    this? The person here said the
    infrared could only arise from
    the bonds vibrating like springs,
    and that only the larger atoms
    had orbitals of energy matching
    the infrared.

  8. Sep 17, 2003 #7
    from your list I assume what gives an object (lets say green paper)its color is the paper absorbing all the light, but then having to emit photons with energies in the green spectrum. Now it seems right (from everyday experience) that the paper will do a #3 and scatter the photons all about, but can it also do a #4 or 5?
  9. Sep 18, 2003 #8

    Chi Meson

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    #4 is stimulated emission, which must happen sometimes in nature, but only by pure accident. But this is what happens in lasers, and in order for this to occur you have to get a bunch of atoms correctly "set up" to release a particular photon if they make a certain jump (a condition known as a "population inversion"). Then one of the atoms spontaneously makes the "jump", and if the photon hits another atom, then two identical photons are created, then they get two freinds, and so on, and so on...
    Getting this population inversion set up is tricky, which is why you do not see lasers occuring in nature. SO will it happen in paper? probably a photon or two every now and then, but nothing significant, and again, purely accidentally.

    #5 (flouresence) you can see by shining "black light" on white paper, or even better "flourescent" paper. An invisible UV photon is absorbed, but other visible photons are emmitted.
  10. Sep 18, 2003 #9

    Chi Meson

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    I wonder where he thinks how IR comes out of the sun? THere are NO compunds on the sun, yet most of what it puts out is IR. (most photons, not most energy)

    In the hydrogen atom (which is the farthest you can get from larger atoms) it's called the Paschen series. As the electron de-exites from higher energylevels down to the third level, all emitted photns are infra red. WHenever the difference in energy levels is less than approximately 1.7 eV, the emmitted photon will be infra red.

    As for bonds vibrating like springs, this is a description of molecular energy (internal, or thermal energy). Sounds like someone has the two very different realms of "heat" mixed up.

    I tried a google on Paschen series and got a few sites, but do a search for "Brackett Series" and you can find Brackett's own paper from 1922. (The Brackett series is for de-exitation down to the 4th energy level in the hydrogen atom, even deeper IR than Paschen)
    http://dbhs.wvusd.k12.ca.us/Chem-History/Brackett-1922/Brackett-1922.html [Broken]

    He doesn't know what he's talking about.
    Last edited by a moderator: May 1, 2017
  11. Sep 18, 2003 #10
    Thanks, Chi Meson.

    Before you take my side I should
    clarify that this was in reference
    to the heat given off by chemical
    reactions.I explained this heat
    by the mechanism of electrons
    changing orbit as a result of the
    general disturbances going on, a conclusion to which I had jumped because I was not aware at the
    time that chemical bonds are sus-
    eptable to vibration at infrared

    I do not know if I jumped to the
    right or wrong conclusion, but
    the reason he claimed it was
    wrong was by saying the major-
    ity of atoms simply do not have
    orbitals that can ever correspond
    to infrared wavelengths.

    Thanks for finding the info for
    me. I will read about the paschen
    series and brackets.
    Much appreciated.

    Last edited: Sep 18, 2003
  12. Sep 18, 2003 #11


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    Greetings !
    Well, Chi Meson basicly mentioned all the main possibilities
    and explained them. I just wanted to mention one I find
    very cool - when a very high energy photon like X-ray
    and higher up the spectrum passes near an atomic nuclei
    it could become (I'm not sure of the right word to use here)
    an electron and a positron - its anti-matter equivalent with
    a positive electrical charge.

    Live long and prosper.
  13. Sep 18, 2003 #12
    Oh, you mean pair-production.
  14. Sep 19, 2003 #13

    Chi Meson

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    OK. I retract saying that "he doesn't know what he's talking about." We are obviously in chemistry here, and I have no authority whatsoever. So considering what I do know: atoms, from hydrogen to uranium, can and do emmit IR photons. They must be "pumped" first though, to push an electron into a higher orbital from which it can jump down, so in chemical reactions I suppose it's unlikely that Hydrogen's electron would find itself in the 5th energy level and then decend only to the 4th or 3rd.

    So...I'm gonna step out of this one
  15. Sep 19, 2003 #14
    Not a problem. I didn't really
    mean to invite support for any
    side so much as to clarify the
    confusion he left me with in
    thinking about lasers.

    So let me be more specific. Can
    a laser emit infrared? If so, can
    you get it to do this by pumping
    it with infrared or do you need
    to pump it with higher frequency
    light to get infrared out? If a
    laser can emit infrared is it
    being emited by electrons falling
    to lower orbits, or is the pumping
    somehow stimulating emission by
    causing the molecular bonds to


  16. Sep 22, 2003 #15

    Chi Meson

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    YES! Wanna buy one? They're relatively cheap and used in many laboratories. I think a HeNe gas laser can be tuned to emit infra-red, but diode IR is cheaper.

    Generally you have to "pump" atoms with a higher energy photon than the one emmitted. With gas lasers you have to find a convenient "meta-stable" state, so it's more difficult. With diode lasers the atom can be pumped "electronically," That is the electrons in the current pump up the electrons in their shells. I don't know too much more about diode lasers actually.

    Google "Infrared Lasers" for a ton of sites
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