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Concentration of photons in a beam of light

  1. Aug 17, 2009 #1

    taylaron

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    Hi, i've been pondering over the properties of a beam of light and i'm curious if the concentration of photons in eg. a laser beam can be calculated.
    To elaborate on this question, let me ask if we can calculate the number of photons in a beam of light x distance long with a given cross section size.

    when I think of a beam of light, I get the notion that the photons that make up the beam are not end-to-end. or are they? when I say end-top-end I refer to the similarities of a liquid compared to a gas.
    -I'll stop here for now.

    Regards-

    Tay
     
  2. jcsd
  3. Aug 17, 2009 #2
    You can do that by determining the energy of the beam
    E=nhf
    where n is the number of photons
     
  4. Aug 17, 2009 #3
    Hi there,

    I don't really get your end-to-end comment.

    Cheers
     
  5. Aug 17, 2009 #4

    ZapperZ

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    1. Assuming that you have a ~monochromatic source, take a photodiode, and measure the beam power. This will tell you the energy per unit time that is striking the photodiode over that cross section (assuming you have the full cross section striking the photodiode).

    2. You know that E=hf, i.e. the energy per photon, for that particular frequency.

    3. It should be straightforward to find # of photon striking the surface per unit time within that cross section.

    Zz.
     
  6. Aug 17, 2009 #5
    And *x/c
     
    Last edited: Aug 17, 2009
  7. Aug 17, 2009 #6

    Andy Resnick

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    Actually, it's not so simple- photon number is non-conserved, and the photon number and field amplitude are noncommuting variables. For an interesting discussion, see:

    http://math.ucr.edu/home/baez/photon/
     
  8. Aug 17, 2009 #7

    ZapperZ

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    But one can also argue that no photodiode is 100% efficient.

    Still, that is what we use to quantify the quantum efficiency of photocathodes/photodectectors, etc.

    Zz.
     
  9. Aug 17, 2009 #8

    taylaron

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    the end-to-end comment refers to how particles can be touching one another or how they can be separated by space. similar to a liquid compared to a gas.

    ----------

    Now that we've established that it is possible (within some accuracy) to count the number of energetic photons per unit time, length, cross section size and intensity im going to evolve my question into some thing more complex.

    Is there a maximum number of photons per unit distance at a given intensity and cross section size? let me rephrase: "is there a maximum intensity of a laser?"
    -im thinking about photons in a physical sense (although probably incorrect) being back-to-back. Possibly defining the maximum intensity of a laser beam.
    any thoughts?

    Regards,
    -Tay
     
  10. Aug 18, 2009 #9

    jtbell

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    I don't think it is meaningful to describe a photon as having a spatial size in the sense that you want to use it here. Try searching the quantum physics forum here for threads about "photon size." I know it's been discussed there several times in the past.
     
  11. Aug 18, 2009 #10

    Andy Resnick

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    I don't understand what you mean.
     
  12. Aug 18, 2009 #11

    ZapperZ

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    1. "But one can also argue that no photodiode is 100% efficient."

    By using a photodiode to measure the power of the light source, I'm assuming that the photodiode is 100% efficient. There is no such thing. So one doesn't have to go to the extent of arguing the photon number isn't conserved. The instrumentation itself already has a higher degree of error than worrying about photon number conservation.

    2. "Still, that is what we use to quantify the quantum efficiency of photocathodes/photodectectors, etc."

    Photodiodes, photodetector, photocathodes, etc.. all have what is known as a quantum efficiency. How many electrons comes out for the number of incoming photons. This is a common number. Hammamatsu uses this in their specs, for example. So obviously, even with the fact that we don't have anything that has 100% QE, such technique of measuring # of photons and determining its QE is still quite valid and useful.

    Zz.
     
  13. Aug 18, 2009 #12

    Andy Resnick

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    I guess I didn't understand why you said those sentences in response to my comment where I summarize the "photons, schmotons" thread...

    I don't see how detection and detectors relate to the statement "photon number and field amplitude are noncommuting variables".
     
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