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Desctructive inteference of coherent light beams

  1. Apr 6, 2012 #1
    say you overlap two identical coherent light beams with a mirror but opposite phase so they effectively cancel each other out, with for instance a partially transparent mirror. what remains in the beam path then? or does such an arrangement mean that the interference forces everything to go the other way?
    I'm wondering if in the place of destructive interference there would be undetectable energy and if that somehow is a novel state of physics or if it's just a matter of a more holistic quantum mechanics interpretation of what's going on.
     
  2. jcsd
  3. Apr 6, 2012 #2

    Dale

    Staff: Mentor

    No, it just means that if the arrangement is even possible it will necessarily generate heat in the mirror.

    http://farside.ph.utexas.edu/teaching/em/lectures/node89.html

    "However, Eq. (1023) is incomplete, because electromagnetic fields can gain or lose energy by interacting with matter. We need to factor this into our analysis. We saw earlier (see Sect. 5.3) that the rate of heat dissipation per unit volume in a conductor (the so-called ohmic heating rate) is E.j This energy is extracted from electromagnetic fields"
     
  4. Apr 6, 2012 #3
    Wave coherence is an ideal property that enables interference...cancelling of wave energy. These mathematical idealizations are limit cases that never occur in reality but allows us understand the underlying physics. Actual waves cancel due to destructive interference but are not perfectly destructive.
     
  5. Apr 6, 2012 #4
     
  6. Apr 7, 2012 #5

    Dale

    Staff: Mentor

    It is true for the scenario you described, and also true for any scenario involving complete destructive interference. You cannot have complete destructive interference in the absence of an interaction with matter, and in that case the missing energy goes into the matter.

    This is not complete destructive interference. In the absence of matter you can get destructive interference in one location, but it is always accompanied by constructive interference in another location. In this case, in the region of intersection the two beams do not interfere destructively everywhere, but there are bands of constructive and bands of destructive interference. The total energy in the intersection region remains constant.
     
  7. Apr 7, 2012 #6
    Dale, it doesn't sound to me like you really know enough to answer the question. you shouldn't pretend to know a subject. it's quite counter productive here. no offense. think about it
     
  8. Apr 7, 2012 #7

    Dale

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    You are free to believe what you wish to believe, but my answers are correct. Go ahead and work out the math for yourself if you don't believe that there are regions of constructive interference also.

    Please read the link I posted above so that you can understand the conservation of energy in EM.

    Also see the description of interference between two plane waves at:
    http://en.wikipedia.org/wiki/Interference_(wave_propagation)#Between_two_plane_waves
     
    Last edited: Apr 7, 2012
  9. Apr 7, 2012 #8
    it doesn't matter if there is constructive interference elsewhere. the question is, what is at the location of destructive interference. it appears to be nothing but both components can survive and separate later.
    it may be one of quantum mechanics' curiosities but conventionally speaking it seems to me that space at that location much have a kind of tension that may not be either electric nor magnetic.
     
  10. Apr 7, 2012 #9

    Dale

    Staff: Mentor

    At the location of destructive interference you get a standing wave rather than a travelling wave. The relationship between the E and B field changes, so that they are no longer perpendicular to each other. It isn't nothing, it just isn't a travelling wave.

    There is no need to invoke QM to understand this, nor any need to add some mystical "tension". This is all completely explained by Maxwell's equations.
     
  11. Apr 7, 2012 #10
    not to sound ungrateful, but that doesn't sound true either : ) although the link was inspiring, thank you.
    maybe it's a kind of standing wave with infinitely long wavelength. I have to think more about it tomorrow.
    do you have a degree in physics? or self taught
     
  12. Apr 7, 2012 #11

    Dale

    Staff: Mentor

    My PhD is in engineering, with my research in magnetic resonance imaging.

    I am starting to feel rather insulted. It is one thing to not believe what someone says without getting some independent confirmation, but now, with no reason, you have twice questioned my truthfulness and also asserted that I am "pretending to know" the subject.

    Frankly, you should either attend classes from a professor you trust or pick up a good textbook that you trust. It is clear that you are not able to learn from an internet forum since you prefer to cast unwarranted aspersions on the character and knowledge of the people you encouter there. You are wasting your time and mine.
     
    Last edited: Apr 7, 2012
  13. Apr 8, 2012 #12

    Cthugha

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    That is hard. You are given the correct and generally accepted (and also taught in standard courses!) answer that destructive interference is always accompanied by constructive interference nearby and that it is impossible to construct a situation where you get complete destructive interference everywhere if you start from a situation with two seperate beams and you give such an arrogant response? You are not likely to get any more answers in these forums if you continue like that.

    Also:
    As you were already told, that is nonsense. They may interfere destructively at the center of the cross section and constructively at the border or the other way round, but not destructively anywhere.

    If you want to understand it in depth think about why any perfect 50/50 beam splitter must necessarily introduce a pi phase shift between the outpput ports and why there can be no ideal "joiner" for optical or radio waves which merges two beams perfectly into one joined beam. Although that may sound unrelated, it is one of the key points to understand why there is no destructive interference without constructive interference (and the other way round).
     
  14. Apr 8, 2012 #13
    In such case if E-field is completely canceled, B-field must gets stronger by constructive interference(you can draw some arrows to see it, and don't forget the right-hand rule of E,B and wavevetor k), there's no way to cancel E&B field simultaneously.
     
  15. Apr 9, 2012 #14
    Dale, hang in there. not buying the first explanation is key to intelligent pursuit. don't see it as an insult. and apparently you aren't a physicist and the question I ask really require someone with very solid understanding of ED and QED.

    what kind of engineer are you? electronics?
    have you built your own working MRI? I may have some questions in that area
     
  16. Apr 9, 2012 #15

    Dale

    Staff: Mentor

    Really? You are honestly going to try to pass "you shouldn't pretend to know" off as "key to intelligent pursuit"? You honestly think that is not insulting or rude at all, just normal behavior for people trying to learn? If so, then why am I not the only one who thought it was out of line?

    An appropriate response would have been, "Can you provide a reference or can anyone else corroborate?" I don't expect people to take my word for it, which is why I posted a reference with my initial response in this case and why I am always glad to provide a reference when asked. I would think that a real "intelligent pursuit" would also include reading reference material provided.

    Frankly, I don't think that you are making any thing close to an "intelligent pursuit". You have a typical faulty understanding of EM but think that you have reasoned out some deep truth, and rather than learn some basic EM you prefer to dismiss the concepts that don't fit in with your pre-existing flawed understanding and the associated idea that felt so profound and meaningful to you.

    No, it doesn't. The question you ask is a very common and basic classical EM question requiring nothing more than Maxwell's equations. It gets asked and answered on this forum every couple of months. Google: "site:www.physicsforums.com destructive interference conservation energy"

    I again recommend finding a good textbook and/or a professor you trust. If you wish, feel free to share my responses with them.
     
    Last edited: Apr 9, 2012
  17. Apr 9, 2012 #16

    Vanadium 50

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    DanFrederiksn, you asked a question and you got a(n) (correct) answer. Insulting the people who answered you is not helping.

    If you are thinking about using this as a jumping-off point for non-mainstream physics, please reread the PF Rules (which you agreed to when you joined) on Overly Speculative Posts.
     
  18. Apr 9, 2012 #17
    Dale, my knowledge of physics is still limited or I wouldn't have asked the question but I have a keen mind and a lot of experience with the dynamics of human psyche, what people know, what people think they know, how well they handle questions outside of prior considerations, how devoted they are to ideas of own standing etc.
    try to leave room for the possibility that not everything I do is in error.

    I asked you what kind of engineer you are. which is it?
     
  19. Apr 9, 2012 #18

    Dale

    Staff: Mentor

    Sure, but your idea on this thread is in error.
     
  20. Apr 9, 2012 #19
    my confidence in your answers is not strengthened by your avoidance of simple questions
     
  21. Apr 9, 2012 #20

    Vanadium 50

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    This thread is done.
     
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