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Destructive Interference of 2 Lasers?

  1. Dec 11, 2015 #1
    If I have 2 identical laser pointers, is it possible for me to point them both at the same spot on a wall and see no light at all due to complete destructive interference?
     
  2. jcsd
  3. Dec 11, 2015 #2

    DrClaude

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    Could you give more details for your scenario? Because even for perfect lasers, you will have to aim them at a slight angle to each other to it the same spot, and this will have consequences.

    Also, are you asking about ideal lasers, or actual devices?
     
  4. Dec 11, 2015 #3
    Actual lasers, but of the highest quality allowed by the laws of physics. Even if both are at an angle, we still can align the wave vectors right?
     
  5. Dec 11, 2015 #4

    DrClaude

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    Unless they are perfectly overlapping, you cannot get a perfect cancellation everywhere. I don't see how you could place the screen such that you get only destructive interference.

    Actual lasers will also have a finite coherence time, so even if the spot was dark, it won't be dark for long.
     
  6. Dec 11, 2015 #5

    Andy Resnick

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    The lasers need to be mutually coherent, otherwise there will be no interference. This can be done by phase locking the sources; for example:

    arxiv.org/pdf/1405.0596
     
  7. Dec 12, 2015 #6

    sophiecentaur

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    Energy is conserved so, if there is destructive interference in one spot, there will be constructive interference somewhere else. The energy just has to go somewhere.
     
  8. Dec 12, 2015 #7

    sophiecentaur

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    Actually, it's worse than that. The only way to get your cancellation 'everywhere' is for the phase centres of the two sources in exactly the same place and for the distribution patterns to be identical. That, I think, would imply that even the emitting atoms would need to be in the same places.
    As usual, I find it worthwhile bringing in the RF equivalent setup. The situation in the OP could involve two transmitting antennae, exactly co-sited, and driven in antiphase, with their feed connections to the antenna in the same place (i.e. the same antenna) will simply produce a zero driving voltage at that point. This would present each transmitter with a short circuit load - which it will not like, haha, and either they blow themselves up or involve an electricity bill for achieving nothing. This idea would also translate to optics and lasers which would be fighting each other just like the two RF transmitters. The practical details are beyond me and the situation is probably unobtainable. I guess an arrangement of half silvered mirrors could easily be made in which no power came out of one port but all the power would have to emerge from the other one. Again, there is an easy RF model which could achieve this.
     
  9. Dec 18, 2015 #8
    You are right, there will always be an interference pattern.
     
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