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Difference between photon frequency and the light frequency?

  1. Dec 17, 2012 #1
    The frequency of a photon is determined by the energy emitted by an electron when falling from one shell to a lower one divided by Planck's constant.

    First, how does a single fall of one electron cause a frequency? I would think the electron would have to jiggle up and down to bring about a frequency.

    Next, what is the difference between a photon's frequency and the frequency of the electromagnetic radiation itself? Are they one in the same? It appears that the photon has an internal frequency of its one, while there is a second frequency that would be how often the photons are created by the falling electrons.
  2. jcsd
  3. Dec 17, 2012 #2


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    There are other methods how photons can be created, too.

    It is not a classical "jump". You have to take quantum mechanics into account, and I don't think there is an intuitive description why you get a specific frequency.

    There has to be a dipole moment between the initial and final orbital. This is not really a periodic motion of the electron, but it has some similarity.

    What do you mean with "frequency of the electromagnetic radiation itself"?
    If you switch a light bulb on and off once per second, is that a "frequency of the EM radiation"? In that case, this frequency is related to bunches of photons: If the light is on, photons are emitted, if the light is off, "no" photons are emitted.
  4. Dec 18, 2012 #3
    I had always thought light was just a classical wave, but made up of these small particles called photons. I then read some descriptions of photons as being red or blue, etc. and whoo!, how can that be, I thought? I then learned that the photons themselves do have a frequency. So, it's all opposite, instead of waves being made of photos, the photons are made of waves and the color is in the photons themselves!

    Then I started studying about the double slit experiment and the de Broglie wave, or what is also called a matter wave or a probability wave. I now understand that photons and all small particles are made up of these de Broglie waves. I'm having a long discussion with a friend about the double slit experiments and he tells me our consciousness collapses the de Broglie waves into more localized wave packets forming real objects. Our consciousness makes things real so to speak. There’s a fair amount of consensus actually, but I counter with the claim that waves are just as important as particles and there is no preference as to which is real, particle or wave. Without these wave characteristic, light could not be focused or makes colors. Particles just need to behave as particles sometimes, and as waves at other times to make our world work. For instance, electrons must behave as waves to remain in orbit in atoms. He countered by saying I'm confusing the de Broglie wave or the probability wave with the electromagnet wave. He says in the case of photos there is a clear distinction between these probability wavea and the overall electromagnet wavea that are defined by Maxwell’s equations. When I research the issue he seems to have a point: "...Electrons manifest this duality: in the two slit experiment their wave nature appears governed by the de Broglie wavelength. Photons do the same too, displaying the Wavelength/frequency associated with the collective classical electromagnetic wave. The classical electromagnetic wave is built out of photons in a consistent way." So there it is; it appears that my friend is correct and my original feeling about light being a traditional wave just constructed out of photons was correct after all. So is there a distinction between the frequency of the photons and the frequency of the overall electromagnetic wave? Is there any real difference between the de Broglie waves and what we normally think of as classical electromagnectic radiation? It seems a bit strange to me that everyday light is made from probability waves. It also seems strangs that colors come from probability waves of where a particle is likely to be found.
    Last edited: Dec 18, 2012
  5. Dec 18, 2012 #4
    The electron does infact jiggle up an down as it "falls" down to the lower energy state. During the "fall" the electron will be in a superposition bewtween the two shells, and if you put up the math for that superposition, you will find that it is not stationary, but rather that the average electron density oscillates with a frequency given by the energy difference between the two states.
  6. Dec 18, 2012 #5
    I'm not sure how exactly you are envisioning the photons. They are quantum objects that are difficult to describe in words or visualise. One simplified explanation could be that photon is a wave in the electromagnetic field with the smallest possible amplitude.

    If you create a lot of photons, each one of them has a frequency f, then the resulting macroscopic electromagnetic wave has the same frequency f.
  7. Dec 18, 2012 #6
    Thank you. That helps me visualize the process much better.
  8. Dec 18, 2012 #7
    Thanks also to you. That helps. So if there were no de Broglie wave, there would be no electromagnetic radiation as well, correct? The electromatic or classical aspects of light is just composed of lots of photons. To put it simply, if there were no probability waves associated with photons, there just wouldn't be any light. What lights my room are literally the probability waves (matter waves, de Broglie waves) of the photons.
  9. Dec 18, 2012 #8
    Well, photon is a wave in electromagnetic field, described by the four-potential Aμ. Probability wave is something more abstract; it is definitely not the same as the wave in Aμ.

    I don't think it is correct to say that each photon has its own probability wave and these probability waves are filling your room.
  10. Dec 18, 2012 #9
    But be very careful with that picture of a photon being the "smallest possible wave". In the one-photon state (or in any state with a definite photon number), the expectation value of the electric (and magnetic) field is zero everywhere.
    A single photon state is a quantum-mechanical superposition of different field configurations with equal probabilities for the field being in +x or -x direction.
    If you want to have a photon state that is close to a classical em-wave picture (a coherent wave like in a laser) this state is a coherent state that is a superposition of different photon numbers.
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