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Question about light waves canceling each other

  1. May 24, 2013 #1
    Hello fellow physicists :)

    I have the following question:

    take a beam of light traveling in some direction x, let's use lasers due to their coherency and monochromatic properties.

    Now let's take another nearly identical beam traveling in the same direction x, only that it's 180° in phase with the other beam.

    Assuming that the beams overlap each other, what would actually happen? Will the cancel each other? And if so, won't it violate the Energy Conservation principle?

    Notice that the beams emanate from the "same" source and travel in the SAME direction, not opposite to each other (then constructive and destructive interference should occur).

    This question is also pointed towards sound waves (Not this forum I guess), meaning: What would happen when two identical yet 180° phase different pressure waves emanate from the same source and travel in the same direction.

    Thanks in advance! :)
  2. jcsd
  3. May 24, 2013 #2


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    Easy. This is equivalent to the source not emanating radiation at all.
  4. May 24, 2013 #3
    That's exactly what I thought, but how do you resolve the issue with light waves?
  5. May 24, 2013 #4


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    Which issue?
  6. May 24, 2013 #5
    The issue is that theoreticly, one could create such a situation with light waves
  7. May 24, 2013 #6


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    I don't doubt this. I just want to say that if you couple two lasers in such a way that the light beams are 180 degrees out of phase they won't laser at all.
    On the other hand I can describe any fieldless situation as a superposition of two fields being 180 degrees out of phase.
  8. May 24, 2013 #7

    Vanadium 50

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    But why do you say that "theoreticly [sic], one could create such a situation"? DrDru has already told you that such a system does not radiate.
  9. May 24, 2013 #8
    Lets take a laser beam, split it into 2 beams, pass one through vacuum and the other one through a gas filled tube, and then recombine them and you get a combination of 2 identical laser beams with different phases, and that I've managed to do in a lab. I indeed got an interference pattern due to the phase change, but my question is what kind of image might I get when both beams are perfectly aligned. I think that the experiment that I've done (It was in order to find the refraction index of different gasses) shows that if the beams are 180° phase different, you won't see any light hitting the screen (you'll get something due to the beams being unaligned).

    I think the answer might be that light is both a wave and a particle, and that the energy keeps moving in the particle form rather than the wave form.
  10. May 24, 2013 #9


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    This question has popped up periodically. Check out this thread:


    I've given, in that thread, a very good paper that explains this conservation issue.

  11. May 24, 2013 #10


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    You need beam splitters to do this. Where recombination takes place, two rays enter and two rays leave. The intensity ratio of the beams leaving depends on the phase of the beams entering.
  12. May 24, 2013 #11


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    You can get total destructive interference and loss of EM energy in the presence of matter, i.e. the waves do work on the matter e.g. the recombiner. In the absence of matter if you have destructive interference in one place you will always get constructive interference in another place and the total energy of the wave is conserved. Here is my favorite proof, it applies for any situation governed by Maxwell's equations:


    The situation you are describing is completely classical. No need to go into quantum stuff.
  13. May 24, 2013 #12


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  14. May 25, 2013 #13
    Thanks people :) I'll be sure to look into those links...
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