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Photon's motion

  1. Feb 16, 2007 #1
    How does a photon move.If it is a particle and moves in a straight line,what do we mean by its frequency in
    E=hf?
     
  2. jcsd
  3. Feb 16, 2007 #2

    Hootenanny

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    No one actually knows since we can't observe a photon without destroying it (i.e. in a photodetector). I'm sure this has been discussed somewhere recently, I'll have a dig around later and see what I can find.
    The frequency is simply the frequency of the EM field oscillations, which determines the properties of light.
     
    Last edited: Feb 16, 2007
  4. Feb 16, 2007 #3
    How do we get E=hf.And so u said that we dont know how a photon travels.its a part of the dual nature of light.So cant it be that the photon a point particle is travelling in sort of a wave?

    What does the de broglie wavelength of a photon and of matter indicate?IS it the wavelength they would have if they were to be waves or sumthning like that?
     
  5. Feb 16, 2007 #4
    That's a very good question but when you ask a physicist they won't give you a direct answer, personally i don't think they don't know the answer.Like if you ask a physicist about an electron's spin it doesn't actually mean that the electron's spinning its something totally different.They'll give you an quantum interpretation of the photon that they are particles of light E= hf the energy acquired by an electron (look at the photoelectric effect) now remembering that a photon can act as a particle or wave hv= quantum energy associated with an e.m wave of frequency v (think of it as a wave) . I wouldn't worry about it too much another way of getting a feel for the photon is by looking at how it also interacts in other situations such as the compton effect ( creation of x-rays) or in the scattering of electrons in a collission it is essentially radiation that is going to interact with matter in 3 primary ways via
    1. the photoelectric effect
    2.the compton effect
    3. pair production
    I hope i haven't bombed u with too much info. Hope this helps!
     
  6. Feb 16, 2007 #5
    yes that is correct if they were waves
     
  7. Feb 16, 2007 #6
    Imagine a simple demonstration: a silly spring(think Americans call them slinkys?) Someone is holding one end and someone the other, now one person introduces some movement at one end and the other person a different movement at the other, this is what superposition might look like of two states. We can see the spring moving in combination of these states.

    Now if someone else knocks the spring from another angle we have another state, that's a wave in a 3 state superposition, there maybe many many states, perhaps even infinite states? Infinite different waves within the super position.

    Now envision that someone comes along and measures the spring, now it behaves like a particle, so the act of measurement has destroyed all the superpositions of the spring or it has decohered.

    Now how much does that reveal about what a photon looks like or how it propagates, given that we have a measurement issue.

    This is the Copenhagen Interpretation in a nutshell.

    And believe me it p's physicists off no end :smile:

    EDIT: how would we know it travels in some sort of wavelike configuration? Young's two slit experiment shows it behaves like a wave producing band like patterns on the screen: as each photon passes through the slits, a sort of interference pattern builds up, of course if we try and detect the photon before it hits the screen, we end up with the photon merely striking the back of the screen as if it was a particle?

    What do you think happens when a single photon is fired at the two slits? :smile: now what happens if we repeat this a thousand times?
     
    Last edited: Feb 16, 2007
  8. Feb 16, 2007 #7
    pair production? what is that?
     
  9. Feb 16, 2007 #8

    Hootenanny

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  10. Feb 16, 2007 #9
    SO a photon is sometimes a wave and sometimes a particle moving in a STRAIGHT line.Have i got it correctly?
     
  11. Feb 16, 2007 #10
    It appears from inference that it travels as a wave, as you will see when you look at the experiments below, but as soon as we try and measure it it appears to behave more like a particle losing whatever state it had, this is decoherence, we cannot really know what it looks like because the act of measurement destroys what we are trying to look at.

    However since it produces interference patterns we infer it travels as a wave, as the back screen shows a dispersal pattern similar to a water wave.

    I.e. If we used dyed the water you would see the same banding as the wave passed through the slits and the waves interfered with each other, this is a good analogy of what is happening.

    These links will make it easier to understand

    http://physicsweb.org/articles/world/15/9/1

    http://physicsweb.org/articles/world/15/9/2

    Feynman's two slit experiment expained
     
    Last edited: Feb 16, 2007
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