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Will two out-of phase lights cancel out?

  1. May 21, 2010 #1
    I'm sure this is a silly question. My understanding of waves would suggest that it should be possible to put two (small) light sources side by side in such a manner that no light energy reaches most observer positions, yet my understanding of electromagnetism is insufficient to confirm this possibility. I've certainly never experienced a phenomenon of something like two LEDs next to each-other canceling each-other's light.
  2. jcsd
  3. May 21, 2010 #2
    Do you know what coherent light sources are?
  4. May 21, 2010 #3
    I will soon :smile:

    So am I correct to assume that this is possible but an atypical phenomenon because all natural light sources are incoherent?
  5. May 21, 2010 #4
    No. Because two different light sources are incoherent, there cannot be an interference pattern.
  6. May 21, 2010 #5
    Ah, alright. Thanks.
  7. May 21, 2010 #6
    This isn't a silly question at all. It touches huge swaths of physics including thermodynamics, electromagnetics, basic conservation laws, and quantum physics.

    In quantum mechanics, a photon (an indivisible unit of light) is it's own antparticle which means it should be possible to cancel out light with light. The trouble is where would the energy go?

    If you spend the time to go through this you will find connections between the second law of thermodynamics and optics through the law of Etendue. This law says (in my own reformulation) that you can't take two beams of light that are side by side and make any optical system that overlays the beams, so this answer says no, you can't cancel light with light.
  8. May 21, 2010 #7
    LEDs do emit incoherent light, which makes it difficult to observe interference patterns. However, it is not difficult to make two sources that are coherent and have a stable phase relationship with each other...an antenna makes it easy, a bit more difficult would be lasers locked to a common reference, or one laser split along two paths. In fact, a laser shining through two slits or pinholes is equivalent to this, and shows the interference pattern you'd get.

    The real answer is that even if you have coherent light sources with opposed phases, the fact that the sources do not originate from the same position will keep them from canceling out at all points. You get interference patterns, just not patterns such that "no light energy reaches most observer positions". With more sources, you can confine the emitted energy to smaller, more sharply defined areas or spread it out, but you can't make it go away, the best you can do is absorb it. And there is no way to emit both from the same location...this is simply equivalent to not emitting anything.
  9. May 21, 2010 #8


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    Agreed. This sort of phenomenon is actually used in cell phone towers, for example. They set up a bunch of towers spaced equally apart in a straight line, with the proper spacing such that the waves radiating parallel to the line cancel out. This way the antennas don't waste energy transmitting in those directions, where there are typically no people. Conversely, the waves transmitted perpendicular to the line of towers, in the direction of population, reinforce each other.
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