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Few basic light questions

  1. Aug 1, 2011 #1
    Hi guys!

    I have a few basic questions about the nature of light that seem pretty simple, but I'm having trouble reconciling them. A few of them have to do with the usual problem of particle/wave duality.

    • In textbooks they frequently talk about a "point source" emitting electromagnetic radiation (in a circle, or a sphere in 3D). But when it emits it, how can you view it as photons being emitted? If the wave is a single photon, then how can a particle go in all directions at once? If it is more than one photon, then it seems like the number of them is arbitrary.
    • White light (say, from the sun, or a candle) seems to usually be described as light with a wide distribution of wavelengths, and most of it coming out not in phase with itself. So let's just look at a single wavelength, from the distribution it's giving off. The light is created by electrons jumping back into lower energy states, millions of times a second, and giving off photons. But then it seems like for any arbitrary wave given off, there should be one nearby that was given off exactly [itex]\pi[/itex] in phase later (because of the massive amount of them). And those two waves would interfere deconstructively. So why do we still see white light? Stupid question probably, but it's been bugging me.
    • So, fermions can't have the same set of quantum numbers. That makes sense to me in a single atom. But obviously ones in different atoms, very far away, can have the same set of quantum numbers. But the probability distribution for an electron in an atom is continuous (though I'm aware it drops off very steeply). So I know you could effectively say two atoms are apart so they can have the same quantum numbers. But then, where do you draw the line? When do electrons start being affected by the exclusion principle?
    • Do wave descriptors like polarization and phase mean anything when viewing light as a photon?

    I'd love to hear anyone's ideas, or any reading you could suggest for me.


    Edit: A bonus question. In all my textbooks, the model they give for an electromagnetic wave has alternating E and B fields, perpendicular and in phase. So at the nodes, E = B = 0. My question is, then, when the wave has reached a node and the fields are zero, what makes them come back up again? Also, at that moment, where is the energy associated with the wave?
    Last edited: Aug 1, 2011
  2. jcsd
  3. Aug 1, 2011 #2
    1) in quantum mechanics, a point particle has a property known as duality, which means in some experiments it acts as a wave, (If you look up the double slit experiment the interference pattern shows light acting as a wave as the peaks of one wave reacts negatively for the troughs of the other wave, producing a alternating light and dark pattern, showing that light is a wave) yet if you try to measure if a particle passes through the inteference pattern disapears and a particle is detected, meaning a wave can act as a particle depending on the experiment
    2) Becouse all the light given off with all the different wave lengths react to make a mix that gives of white light
    3) Depending on the energy they have, if they have more energy then they can get close, but soon it is impossibe to get any closer
    4)Polarization works when light is a point particle if you have a group of photons
  4. Aug 1, 2011 #3
    Err...I don't think you really answered any of my questions. Did you read them?
  5. Aug 2, 2011 #4
    Can anyone else help me?
  6. Aug 2, 2011 #5


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    • Photons are emitted in random directions from the point source. Each photon only goes in one direction, a single photon does not spread out everywhere.

      Light does not interfere with other light very much. In a double slit experiment the light is interfering with itself in the slits, not other light. In addition, two electrons that fall from a identical orbitals to another identical orbitals do not give off exactly the same wavelengths of light due to the uncertainty principle. But they are so close there is almost no difference in the two.

    I'm not totally sure, but I believe that the dividing line would be when the wavefunctions become identical. Two electrons can overlap in orbitals or possible positions, but their wavefunctions are NOT the same. Only when the wafefunctions become identical do they matter. Also, these wavefunctions also have to occupty the same spots in space. (IE they need to be in the same atom generally)

    Is a vibrating string that has returned to 0 amplitude at 0 energy? No! It continues to move back and forth. A similar effect is happening here I think.
  7. Aug 2, 2011 #6
    Thanks for replying!

    Ok, but my question is then: for a single pulse, how many photons is that? How is that number decided? How about this scenario: A charge, q+, is accelerated one full period, back and forth. Only once. So that should give off some kind of EM radiation, right? What's the nature of that radiation?

    Wait...I thought in the double slit experiment, the interference is at the screen, not the slits themselves..? I might be wrong.

    Hmm, unfortunately a vibrating string is a standing wave, not a traveling wave like a regular EM wave is. Also, a particle at a node (in a traveling wave again, at a moment in time when the amplitude at that particle is zero) has the most kinetic energy it will have during its cycle.

    From an old into physics textbook: "In general, the electric flied magnitude E is a function of position and time; thus the energy density u of an electromagnetic wave, given by [itex]u = \epsilon _0 E^2[/itex], also depends in general on position and time."

    They seem to be touching at something, but not going any deeper, because they don't address an obvious question that comes out of this equation: Energy must be conserved. If u is a function of time, that means that the volume (volume? Of what? The book says "...the total energy density u in a region of empty space where E and B fields are present") must be changing. More problematically, that means that when E = 0 (at the node), u = 0, so the volume must go to infinity at that moment. Is there some dirac delta function magic happening here? Or something with the uncertainty principle I'm not seeing?

    Anyone else?

    Last edited: Aug 2, 2011
  8. Aug 2, 2011 #7


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    I can't really help you much, as I am not familiar enough with the material.

    I think interference happens, or at least starts, at the slits. This meaning that if you place the detector anywhere after the slits you get interference, but anywhere before it you don't.
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