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Frequency Of Light Question

  1. Jul 18, 2004 #1
    I am starting to slowly read QED, by Richard Feynman.

    He is quite adamant that light is particles, and not waves, and he cites the evidence of the action of the device called the photomultiplier to explain his stance. I find this argument completely convincing.

    However, looking at the photon as a particle causes me some confusion whenever he, or anyone, mentions the frequency of light.

    Viewed as a particle, I have no idea whatever what is occuring at a given frequency to say that it even has a frequency.

    Sometimes I wonder if this means that so many particles per unit time pass a given point. At other times I wonder if each individual photon possess its own frequency in some regard: is it spinning at a certain number of spins per unit time, or is it, maybe, undulating somehow a certain number of times per unit time?

    So, my question is "Does an individual photon have a frequency, and if so, what is it doing at that frequency?"

  2. jcsd
  3. Jul 18, 2004 #2
    If you believe in string theory then a single photon will have frequency.but if you stick to classical physics you will will encounter the wave/particle duality.
    this says that light has both wave and particle properties and many experiments have reinforced this especially youngs double slit.
  4. Jul 18, 2004 #3
    The frequency is defined as the change of quantum mechanical phase per unit time. That is to say, the quantum mechanical phase associated with the amplitude for the state, photon, electron, etc.. Analogously the wavenumber is the QM phase change per unit length. The are associated with the classical quantities energy and momentum, which combine to make a fourvector, the fourmomentum or fourwavenumber. The factor of conversion is h, Planck's constant.

    It will get a good deal more interesting and complicated before you really begin to understand what it happening. The waves interfere with each other constructively and destructively in space and time, e.g. in the two slit experiment, producing effects which are not describable by classical mechanics.
  5. Jul 18, 2004 #4
    In 'traditional' quantum theory, if you like, i.e. before De Broglie, Schroedinger and wavefunctions etc., each photon has a frequency, which manifests itself in the energy of each photon. If the light has a higher frequency, it means that each photon is more energetic, -not- that more photons pass per unit time. This (more photons passing) means that the light has greater intensity.
  6. Jul 18, 2004 #5
    limited wave

    In my book on early QM they used the classical description of a wave bunch to get the idea over . If you take several sinewaves and add them together( especially ones which have harmonic frequency reations) the effect is to produce a wave form with a modulated amplitude which looks like bunches of waves ( the bunches are repeated). They then go on to imagine an infinite set of sines with a limited bandwidth , the results are that you end up with one wave bunch of a given length and shape. This bunch has an average frequency related to the group chosen. The maths is based on the fourier transform -- standard maths. This bunch however exhibits properties similar to the quantum mechanical concepts of property pairs momentum versus position etc and in fact can be used to deduce an approximation for h Planks constant. Although limited the model lets you ask questions such as -- how long is a photon-- it lets you picture more energy by showing how higher frequencies results in more waves in a bunch , it allows for self interference over some length range which lets you picture the double slit experiment ( however inaccurately).
    I am not sure if this analogy is still used, QM has come a long way, but it does help envisage a particle ( localised) with wave properties and the maths is real.
  7. Jul 19, 2004 #6
    Thanks everyone. These answers have been pretty helpful. I am clear now about the fact that the frequency of light has nothing to do with number of photons passing a given point per unit time, which is a good notion to be rid of, and that it does have to do with something the photon itself is doing at a certain frequency: changing its Quantum Mechanical Phase.

    The rest will take me some time to mull over and sort out.


  8. Jul 19, 2004 #7


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    Not phase but energy and momentum. For a massless particle in relativity they are numberically equal. The frequency is the momentum divided by Planck's constant.
  9. Jul 19, 2004 #8


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    From your knowledge of bosonic string theory, does the string representing the photon operate in the curled up dimensions? It seems like maybe it only has to be in 4-d spacetime. Does the photon frequency (or momentum) directly relate to any string parameter?

    Thanks in advance

  10. Jul 20, 2004 #9


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    The oscillations of the string are what (are supposed to) generate the particles we experience. The string oscillates in all the transverse dimensions (that is the dimensions not internal to its world-sheet). This includes the compacted ones. There are exceptions where the string has its endpoints on two different branes and the branes are in various configurations.
  11. Jul 21, 2004 #10
    yes but what if it is all wrong... i know i know... but what if all photons from a standard light source weren't identical... what if they had a range of speeds and the result of viewing tham all was a white light. i think that if we measured light for specific speeds we would find slow light and fast light. i don't have much of a reference for this idea but my own, but it fits, and explains alot. especially when put to the test by the "red shift-blue shift" of a black hole.
  12. Jul 21, 2004 #11
    I would imagine that someone has done this. Since the invention of the laser it is possible to have light all of one "color" i.e. frequency to test. I haven't heard of it, but I would be surprised if someone hadn't already checked to see if there is a different speed noticable for different specific colors.

    It seems, too, pretty obvious from what has been pointed out in this this thread, that all photons are different from each other in many regards. (The single regard in which they aren't suspected of differing is the speed at which they all propagate.)
    Last edited: Jul 21, 2004
  13. Jul 21, 2004 #12


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    [Exact citation: B. Schaefer, PRL v.82, p.4964 (1999).]

    To date, that is the most accurate measurement of light of different freq. This means that we have no experimental evidence using the best technique we have so far of validating your "what if's".


    Addendum: This just came out. A new expt. measurement of the speed of light at extremely low freq., in the range of 5 to 50 Hz![1] Again, no detectable deviation, and based on the accuracy and limitations of the measurement, this puts the upper limit on any possible photon rest mass at less than 4 x 10^-52 kg (!!). This number keeps getting smaller and smaller with each subsequent refinement and improved measurement.

    [1] M. Fullekrug, PRL v.93, p.043901 (2004).
    Last edited: Jul 21, 2004
  14. Aug 16, 2004 #13
    Adding Arrows

    How, in practical terms, did Feynman figure out the beginning and ending direction of the stopwatch pointer? The concept is clear to me, but I don't see how anyone could accurately time a photon over such a short distance at such an enormously fast frequency. What is the trick to this?
  15. Aug 16, 2004 #14
    I am familiar with Fizeau and MM, but it occured to me that some different means of measuring the speed of light must be used for the very faint light from stars and events in far space. How do they gather such light, and measure its speed these days?
  16. Aug 16, 2004 #15
    I also started reading QED a few days ago and I think you misunderstand what he was doing with the arrows. He wasn't measuring the speed of light, he already knew that. And since he also knew the distance, he could calculate the time. That's how he knew how far the clock hand would move.

    Please correct me if I'm wrong, I'm just learning this stuff too.
  17. Aug 16, 2004 #16
    OOPS, I meant to quote post#13.

    I dont think the starting position of the clock is important as long as it's used for all arrows, it's the difference between arrows that gives the result.
  18. Aug 16, 2004 #17
    Please. I KNOW he's not measuring the speed of light.
    This is what I'm asking: how could he know the distance when a photon might take any possible path? Any photon that refelected and picked up by the photomultiplier may have reflected off the front or the rear surface. He implies that he is able to time the photons and derive the length of the path from the time.
  19. Aug 16, 2004 #18
    Oh sorry, we must be reading different books.
  20. Aug 16, 2004 #19
    All photons of length <15 nm to >9000 nm are particles

    The range noted in the title dwarfs the visible range of ~ 400 nm to 900 nm. It should be remembered that, for hundreds of years and long before the velocity of light was measured, spectral emissions were known only by
    their lengths; only after "c" was known, was it possible to compute a pseudo-frequency: f = c/photon length. On the contrary, Faraday/Maxwell AC radio transmissions rightly have a reciprocal relation because they are truely electromagnetic waves.

    Incidentally, although RPF's "QED" includes a 40-page Chap 2 (Photons: Particles of Light); the second chapter of Vol I of his lectures, page 2-5, shows that light is listed with the acknowleged wave phenomena of Radio Radar etc - of course it should have been listed with his particles group that included UV-, x- and gamma- particles (i.e., not rays). Cheers, Jim
  21. Aug 17, 2004 #20
    What do you mean by the term "spectral emission"?
    What do you mean by "pseudo-frequency"? Are you saying the frequencies we ascribe to light are guesses or estimates, or are you saying the whole concept of light having a frequency is "pseudo"?
    What do you mean by "Faraday/Maxwell AC radio transmissions"? (It's the "radio" part I'm wondering about. Obviously those two didn't work with radio.) And a reciprocal relation to what?
    Yes the whole issue of EM waves vs photons(light) is one I was going to ask about specifically at some point. Maxwell, still aether-bound, asserted they were two related disturbances in the same medium, but not that they were exactly the same thing. Einstein, if I recall correctly, believed there was a kind of threshhold beyond which EM became photons, but I don't know the details. They both arise in the electric field, but I'm wondering how clear anyone is about the cut off, or threshold, between the two. I have been under the impression that as low as infra-red we already have photons.
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