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Light Intensity and Number of Photons

  1. Nov 29, 2009 #1

    If light intensity is determined by the number of photons, what determines the number of photons?

    A higher energy wave does not mean more photons, right?

    A higher energy wave just means that the photons have more energy, because higher frequency means more energy?

    How come a shorter wavelength does not mean it has more photons than a wave with a longer wavelength?

    I am confused.
  2. jcsd
  3. Nov 30, 2009 #2


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    Light intensity is not exclusively determined by the number of photons. Light intensity depends on both the photon flux and the energy of each photon.
    The use of "wave" here isn't really appropriate. You can think of light as a wave or a beam of photons, but not both. Photons to do travel in waves. So if we rephrase your question as "A light beam with higher intensity does not mean more photons, right?". Correct. Continuing from my previous point, a higher intensity can result from either an increase in the number of photons (per unit area), or higher energy photons. Or any combination of the two.
    See my previous point.
    See my previous point.
  4. Nov 30, 2009 #3
    No the previous is not correct. Higher intensity means more photons per unit area.

    Think of two lasers. A red laser and a blue laser.

    The red laser shoots out 20 photons on a area of 1cm2 while the blue shoots out 10 photons

    Which one has a higher intensity? the red laser because it has more photons per square cm

    Which one has a higher energy? the blue laser because the blue photons have a shorter wave length therefore higher energy

    Those two are definately not interchangable
  5. Nov 30, 2009 #4


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    This and your reasoning afterwards is wrong. The number of photons per second per unit area is given by the photon flux.
    Intensity, however, is the irradiated power per unit area and therefore equal to the photon flux times the energy of the emitted photons.
  6. Nov 30, 2009 #5
    I am sorry, i have made a mistake. I was thinking of the photon flux.... my bad
  7. Nov 30, 2009 #6
    Wait, so if light intensity can be determined by the energy of the photons, doesn't that mean light intensity depends on the energy of the light, since the energy of a photon is determined by the frequency?
  8. Dec 1, 2009 #7
    Light intensity is determined by the total energy or work done over an area, So if you have lets say 10 photons with with energy 10joules each hitting an area over 1 second then you will have 100 watts/unit area right? but if you had 5 photons with 20 joules each then you will still have the same 100 watts/unit area but you have simply traded off between number of photons and the energy of each photon.

    Intensity is not just determined by either the number of photons or the frequency(energy) of each but a combination of both. I dont know if you now this but there is a difference because the most powerful laser in the world is not the most intense laser in the world..... hope this helps

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