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Understanding what Light is (My attempt at an explanation)

  1. Jul 22, 2011 #1
    I have to do an essay for a composition class where we explain something "I don't quite know everything about but feel as though I have a firm grasp of" and I was originally planning to do an analysis of sorts of a post-modern literary work but my professor thought it would be more in line with my other papers if I did something I was working on in Physics.

    Though I'll be starting some decent physics this fall (Modern and directed studies in QFT) I'm still concerned about my basics so I decided a few weeks ago to start reading through some basic texts to check my understanding of topics. I've been reading through Crowell's book on Physics and Hewitt's on conceptual Physics (If you hit this thread Mr. Crowell, I really like the book!) and watching Susskind's lectures (though I'm not that far in any of these yet since it's a bit to do at once).

    I decided that I would try to explain what Light is. We read a passage that was from the 1920s that described why the sky is blue and I figured I could try to figure out a reasonable explanation of what light is on basic conceptual level.

    My goal is to try and discuss the different perception people have of light a little and try to better explain the idea that light is "both a particle and a wave" etc. This is largely because I feel that this is done poorly for most people not only because not everybody reads their books and some books are just awful (I'm very grateful to have two solid conceptual books to refer to) but also because some teachers don't really understand themselves. My own high school Physics teacher knew her Classical Mechanics extremely well and the same went for her Classical E&M but anything beyond that was pushing her comfort zone.

    Anyway, I came here to ask if anyone would like to read it and see what they think. If anything is wrong don't hesitate to point it out, but please have a solid explanation or a reference at your disposal, I'd like to learn something from this. I'm not looking to have people point out issues with commas or anything, this is solely content oriented. If someone feels this needs to be HW I'll move it but I'm not really asking for help with the writing assignment, just in seeing if I understand the ideas well enough to explain them to someone else.

    If any interest is expressed I will likely post a copy of the draft later tonight.

    Thank you in advanced for your time,
    Elwin
     
    Last edited: Jul 22, 2011
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  3. Jul 22, 2011 #2

    ZapperZ

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    Have you read the FAQ in this forum? You might want to take a look at it.

    Zz.
     
  4. Jul 22, 2011 #3
    I'll read it again, ah, I'm guessing you're referring to:

    Sorry about that, I didn't see the part about anything related to school work, I figured since it wasn't Physics HW and since I'm not really asking for work on the essay's mechanics it would be okay but I have no problem moving it.

    Can I just copy my post and then delete the thread or what? I don't know exactly how to move it otherwise.
     
    Last edited: Jul 22, 2011
  5. Jul 22, 2011 #4

    ZapperZ

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    Er.. no, I don't mean the STICKY. I mean that there's an FAQ Subforum. One of the question tackled is the wave-particle duality.

    Edit: Oy vey, you shouldn't have deleted your original post.

    Zz.
     
  6. Jul 22, 2011 #5
    Oh wow, I didn't see that, sorry.

    I've read through it now and I think that how I'm trying to describe Light is in agreement with the FAQ except for one bit that I don't know how to show yet (the QED approach is equivalent to the Schrodinger Equation approach; though, I had heard that Feynman derives the Schrodinger equation from his formalism in his book with Hibbs).

    Sorry for the misunderstanding, I'm not very experienced with forums and I didn't want to put something in the wrong place.

    Thank you for directing me to the FAQ though, my elementary assessment appears to be in agreement with what is posted there so I guess I'll work on revising it to be more easily read.

    Thanks again,
    Elwin
     
  7. Jul 22, 2011 #6

    HallsofIvy

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    One thing I did once with a basic physics class was this- one student was given a small red rubber ball I stuck onto a short stick. "That's an electron- and electron on a stick!". I gave another student another small red ball but attached to a string. "That's an electron on a string!"

    Now, we know that electrons, being of like sign, repel. That means that the "electron on a sticK", which cannot move, will make the "electron on a string" move a tiny bit back from hanging straight down. Now move your "electron on a stick" a little bit toward the "electron on a string". Because the distance has shortened slightly, and the force depends on "1/r^2", the force is slightly increased so the "electron on a string" moves slightly farther back. If you swing the "electron on a sticK" back, the force is reduced and the "electron on a string" moves forward again. If you swing the "electron on a stick" back and forth, the "electron on a string" will swing back and forth at the same frequency.

    That's what light is. Every electron affects every other electron- we say it has an "Electric field". Light is changes or "ripples" in that electric field.
     
  8. Jul 22, 2011 #7

    ZapperZ

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    But I think you might be missing the point here.

    While it isn't exactly "wrong", using the wave-particle duality perpetuates the implicit idea that light has two different descriptions, and that we switch gears when we describe it from having wave properties to particle properties. This is not true, and that is what the FAQ tries to convey. QM has one, consistent description for ALL the phenomena of light, be it wave-like or particle-like. Unlike classical physics where there is a dichotomy between particle and wave, this is not there for QM.

    So now, do you call it a wave, or a particle, or do you want to call it both?

    Zz.
     
  9. Jul 22, 2011 #8
    The way I'm understanding it is that light is comprised of discrete packets of quantifiable energy that exhibit behaviors similar to those of a particle when collapsed and otherwise are just unknown distributions of energy.

    Or is this incomplete/flawed? I don't think of light as both a particle and wave...I think it's something that I don't have a concrete noun for that acts as I tried to describe above

    Thank you for your help,
    Elwin
     
    Last edited: Jul 22, 2011
  10. Jul 22, 2011 #9
    That's roughly how I answer my section on describing what it's "waving" in since apparently it's a fairly common question.

    I've done enough E&M to (think that) I know that light is a propagation of energy in an electric field and that the variations in the electric field produce another transverse magnetic component that is orthogonal to the variations in the electric component.

    Unless I'm getting this wrong too, which I'll admit I wouldn't be shocked by. I've skated through Math and Physics on Calculation based skills for a long time now.
     
  11. Jul 22, 2011 #10
    The way photon's behave is described by quantum electrodynamics. The point that ZapperZ is trying to convey is that this is the correct description of all phenomena, that is QED. Now, classically we always observe light propagating as a wave. Where does this come from? There are certain states described in QED that have large number of photons, but that number must also be an indefinite number of photons for there to be an electric field. So in certain limiting cases of the theory of photons, the electromagnetic field behaves as a classical EM wave. Then in certain other cases, for example atomic electron transitions, the photon behaves something like a classical particle. But the photon is neither a wave nor a particle as those words are defined in the context of classical physics. A photon is a quanta of the electromagnetic field.
     
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