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Photon Interactions

  1. Feb 15, 2005 #1
    I read that photons can interact with electrons, atoms or simply disintegrate. The four cases I came across are.

    1. Compton scattering.
    2. Photoelectric effect
    3. Knock an electron to a higher energy state in an atom.
    4. electron-positron pair production.

    While I understand each process what determines when one would occur as opposed to another. For example, under what conditions does Compton scattering occur vs. the photoelectric effect?
     
  2. jcsd
  3. Feb 15, 2005 #2

    jtbell

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    In general, when a photon interacts with matter, any of the processes you name can occur, so long as it doesn't violate any conservation laws. For example, pair production can occur only if the photon has at least enough energy to create the masses of the electron and positron, namely 1.022 MeV.

    Otherwise, a photon with a given energy, interacting with a given kind of material, can interact via the photoelectric effect with a certain probability, via the Compton effect with some other probability, etc. These probabilities are usually expressed via a quantity called the "interaction cross-section" for each process. They can be calculated using quantum electrodynamics.
     
  4. Feb 22, 2005 #3
    5. photon/photon interaction. in the visible light band, responsible for color & "whiteness".


    TRoc
     
  5. Feb 22, 2005 #4

    dextercioby

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    What...?Please give details.What do you understand by "photon-photon interaction"?

    Daniel.
     
  6. Feb 22, 2005 #5
    Dex,

    According to "additive color" theory in "the textbooks" they are:

    red+green+blue=white
    magenta+yellow+cyan=white
    red+cyan=white
    blue+yellow=white
    magenta+green=unknown
    red+green=yellow
    blue+green=cyan
    etc., etc.


    TRoc
     
  7. Feb 22, 2005 #6

    dextercioby

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    And what theory is that...?It's definitely not the one that explains color combinations in painting (red +green is never yellow)...And what's the connection to "photon-photon interactions",if any?

    Daniel.
     
  8. Feb 22, 2005 #7

    jtbell

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    That stuff has nothing to do with photon-photon interactions and everything to do with the physiology of vision. In particular, it's connected with how light of different frequencies stimulates the cone cells in the retina of the eye, and how the brain processes the signals. Or is it the rod cells? I can never remember which is which. :confused:
     
  9. Feb 22, 2005 #8

    Hans de Vries

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    There is nothing what can be called photon-photon interaction here.

    These are the additive rules for color combinations. These perceptions
    take place in our brain's neurons only, by combining the inputs from the
    red, green and blue sensors.


    Regards, Hans

    BTW, magenta + green = white and not "unknown"
     
  10. Feb 22, 2005 #9

    Hans de Vries

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    Painting is subtractive (white - red - green = blue)
    not additive (red + green = yellow).

    Regards, Hans.
     
  11. Feb 22, 2005 #10
    Dex.

    As Hans pointed out, the additive theory that I referred to is for light (photons). Why would I bring up paint (subtractive)?

    Hans

    I'm aware of the physiological aspects of light and color, but not talking about them.
    However, the aspects of frequency interaction that regulate the stimulation of cones are there because the eye has evolved around making use of EM frequencies known as "visible light". These aspects are inherent in all frequencies. If photon/photon interaction were impossible, how would you produce the coherent resonance known as L.A.S.E.R.? When I said "the textbooks", I am referring to Physics, and not Biology.

    My other reason for additive in quotes was because it is flawed. One of the big ones is magenta + green. Magenta is a color to which no frequency can be assigned. It does not exist in the currently used E-M spectrum chart, and therefore, cannot be added to anything.

    Newton postulated that "white light contains all colours". This has not been rejected by modern science, and the color wheel for light mixture uses magenta because the subtractive theory uses it flawlessly (color tv, printers, etc.) and must be logically inverse to the additive theory. But again, there is no evidence of a frequency of light that can be described as magenta in any scientific text.

    TRoc
     
  12. Feb 22, 2005 #11

    EL

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    Maybe this should be moved to the "art-forum"...
     
  13. Feb 22, 2005 #12
    Really, this is a bunch of BS.
     
  14. Mar 2, 2005 #13
    el, m.tens

    Faced with such overwhelming evidence..

    proof 1. "Maybe this should be moved to the "art-forum"...
    proof 2 "Really, this is a bunch of BS."

    ..I should abandon logic.

    A laser is possible because of photon/photon resonance. An interaction capable of resonance, must also be capable of dissonance. This is why some colors reflect certain colors, and absorb others.

    If you wish to eliminate evidence base on the neurons "perception", then you must also eliminate all symbols, numbers, and words, as they are just combinations of light and dark that trigger the cones in the eye, and then "perceived" by the brain to mean something. In other words, it is a hollow argument; please try again to convince me that this is not an interaction that takes place outside of the brain.

    TRoc
     
  15. Mar 2, 2005 #14

    EL

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    Photon-photon scattering takes place (or more precise, should according to QED take place) through intermediate virtual electron-positron pairs, so photon-photon interactions does indeed exist. However, what does that have to do with lasers? And what does it have to do with colors?
     
  16. Mar 2, 2005 #15
    EL,

    By "colors", I just mean a specific frequency. I don't see why it bothers some folks. If science had a precise definition of each color (by frequency), this wouldn't be so difficult. I could then say, for example, a wave of 730nm, and everyone would know that I was refering to a NIST standard "red" photon.

    My first post was simply that there was also photon/photon interactions. It was after being questioned that I eleborated on what I consider to be "elementary" physics.

    Lasers are monochromatic, which by definition means "1 color". They started with a red laser, then were able to produce the more refined (expanded data - cd's) green. Blue is next...
    is that enough of a reason to mention "laser" and "color" in the same post? A laser is not 1 photon, but a beam of many "interacting" coherent (resonant) photons.

    Another example? A quote from Encyclopedia Brittanica:

    "In addition to saturation spectroscopy, there are a number of other techniques that are capable of obtaining Doppler-free spectra. An important example is two-photon spectroscopy, another form of spectroscopy that was made possible by the high intensities available with lasers. All these techniques rely on the relative Doppler shift of counterpropagating beams to identify the correct resonance frequency and have been used to measure spectra with extremely high accuracy. These techniques, however, cannot eliminate another type of Doppler shift. "

    Thank you for not getting "personal", after all this is a Forum. I'm here to expand, not contract!

    TRoc
     
  17. Mar 3, 2005 #16

    EL

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    Yes, I understood that.

    What do you mean by interacting? Where does an interaction between photons take place in a laser?
    Usually we differ between "interaction" and "interference".
     
  18. Mar 3, 2005 #17

    Claude Bile

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    Your assertion that photons interact with one another in a laser is incorrect. You cannot get a laser without a lasing medium, you need atoms to facilitate the interactions that would lead to lasing. It is the atoms that posess resonances, not photons.

    What do you mean by 'colour'. Do you mean the frequency of a photon, or the absorption spectra of a particluar substance? Objects appear to have colour because they absorb certain frequencies and reflect others, this property is entirely dependant on the characteristics of the atoms/molecules and not the photons.

    This is incorrect, all lasers posess a finite linewidth, some lasers (supercontinuum lasers) have linewidths that can cover a large portion of the frequency spectrum.

    This does not support any of your arguments. Two-photon spectroscopy works essentially because of what happens to an atom when two counterpropagating photons are incident upon it at the same time. Again, this has nothing to do with photons interacting with one another, it is photons interacting with atoms.

    There are certain interactions termed photon-photon interactions, however they all occur inside media, because they rely on the presence of a nonlinear polarisation. So called 'photon-photon' interactions also require the presence of an atom, so technically, it is a 'photon-photon-atom' interaction.

    I am not aware of any interaction that occurs between two photons, and only two photons.

    Claude.
     
  19. Mar 4, 2005 #18

    EL

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    You've got my point Claude Bile...
    However:

    Photon-photon interaction is in fact possible without the presence of a medium. Or more correctly: Photon-photon scattering is predicted by QED, but has yet not been measured due to the small cross-section. To lowest order it is described by the Feynman diagram with two incoming real photons, a square of virtual fermions, and finally two outgoing real photons.
    It is possible to via an effective-action approach rewrite this in terms of a polarization of the vacuum, giving rise to non-linear corrections to Maxwell's equations.
     
    Last edited: Mar 4, 2005
  20. Mar 6, 2005 #19
    Claude,

    Of course you are right, one can not talk about 2 photons without the 2 atoms they came from. I am no expert in lasers, and have only read the term "monochromatic" used to describe their frequency. Do you know by how many nanometers they can range and still function?

    I'm sure that this has not been done, but what do you think would happen if 2 photons of the same phase and frequency were "fired" simultaneously through a single slit? Would you get an interference pattern?

    Also, can photons experience superposition?

    TRoc
     
  21. Mar 6, 2005 #20

    Claude Bile

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    With regard to your first question:

    Supercontiuum, short-pulse (femtosecond) lasers can have linewidths (bandwidths) in excess of 100 nm.

    With regard to your second question:

    If you fired two and only two photons you wouldn't get an intereference pattern, you would just get two localised events. If you sent many pairs through one pair at a time, you most certainly would get an interference pattern.

    A neat experiment I did in my undergraduate coursework was a standard two slit diffraction experiment, except the interference pattern was measured using a photomultiplier, and the whole setup was shielded from any ambient light. Essentially what we did was attenuate the laser beam until the power was so low, only one photon (on average) existed in the box at any one time, and showed that an interferance pattern is still obtained. In conclusion, an interefence pattern is obtained when you send one photon through at a time, much less two.

    Claude.
     
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