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Help understanding light intensity

  1. Oct 4, 2011 #1
    I have a problem understanding the intensity of light. Lets assume a monochromatic light of a single wavelength. The energy of the wave is constant and equal to h* f where h is the Planck's constant and f the frequency of the wave. The intensity of this wave is its energy divided by a given area.
    What I can't understand is how two sources emitting the same monochromatic wave to the same area can have different intensity. The area is the same, h is stable and if we change f then we talk about a different wave.

    What do I miss?
  2. jcsd
  3. Oct 4, 2011 #2


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    Are you trying to relate the answer to the energy of a single photon? Would there just be a single photon?
  4. Oct 4, 2011 #3
    I will explain my way of thinking and tell me where I am wrong.

    1) The electromagnetic radiation can be either presented as a wave or a particle. An electromagnetic wave of 600nm, for example, means that there is a wave with each creast abstain 600nm from adjacent crests or a series of photons abstain 600nm from adjacent photons. Correct?

    2) Each photon (or crest) has energy equal to (its frequency)*(Plunck's constant). Correct?

    3) This wave travels with the speed of light. Which means that from a surface, each second, the ammount of photons that fit in 299792458 meters are passing through. So, if we have a source emitting 1 meter wavelength photons then 299792458 photons will pass through this surface in a seccond. Correct?

    4)if 3 is correct this means that the intensity of every 1 meter wavelength radiation is stable for a given surface but this is wrong. There might be millions of sources emitting this radiation in different intensities. What am I missing?
  5. Oct 5, 2011 #4


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    A photon is not a "crest" of a wave. This is not an accepted model. Read further about it.
  6. Oct 5, 2011 #5
    Is a photon an EM wave (constant wavelength) with length equal to the speed of light?
    Can we model a photon like this?
  7. Oct 5, 2011 #6
    Although the photon is an entity and it cannot be divided, in contrast to the wavelength.
  8. Oct 5, 2011 #7


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    I think you need to pause and then 'find out' rather than making up your own possible models. For a start, how can a waveLENGTH equal a SPEED? I have no idea how to put you right on this by using the Q and A method. It's so inefficient and far too demanding on my time (and that of others).
    It's not that I don't want to help but it's just impractical. I'm sure it didn't (/doesn't) work that way in your School.
    Just look at the wiki pages if you want to make a start.
  9. Oct 5, 2011 #8


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    The intensity that you mention is actually more usually called the "flux" (at least that's what I'm used to as an astronomer). The energy of a SINGLE photon is hf, however, there can be a variable number of photons impinging on a certain area during a given unit time. The energy impinging on that area per unit time is then Nhf where N is the number of photons. N is the number that changes to give you variable fluxes.
  10. Oct 8, 2011 #9

    Claude Bile

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    Matterwave nails it.

    Your definition of intensity is erroneous;

    1. The energy of the wave is not h*f; the energy of a photon is h*f. The total energy of a wave is h*f times the number of photons.

    2. Intensity (or more correctly, irradiance) is power/area not energy/area. Energy/area is called the fluence.

    Two sources with the same wavelength over the same area can have different irradiances by emitting a different number of photons per second.

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