Destructive Interference of 2 Lasers?

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Discussion Overview

The discussion centers on the possibility of achieving complete destructive interference with two identical laser pointers aimed at the same spot on a wall. Participants explore the theoretical and practical implications of laser coherence, alignment, and interference patterns.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether complete destructive interference can occur when two lasers are aimed at the same spot, suggesting that perfect overlap is necessary for cancellation.
  • Another participant notes that even with high-quality lasers, aiming them at an angle will affect the interference and that perfect cancellation cannot be achieved everywhere.
  • Concerns are raised about the finite coherence time of actual lasers, implying that any dark spot would not remain dark indefinitely.
  • One participant introduces the concept of mutual coherence and phase locking as necessary conditions for interference, referencing external literature.
  • Another participant discusses the conservation of energy, stating that destructive interference in one location must result in constructive interference elsewhere.
  • A comparison is made to RF antennae setups, suggesting that similar principles apply to lasers, with practical challenges in achieving the desired interference pattern highlighted.
  • It is acknowledged that an interference pattern will always be present, indicating that complete darkness at a point is unlikely.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of achieving complete destructive interference with lasers, with no consensus reached on the conditions required for such an outcome.

Contextual Notes

Participants discuss limitations related to coherence, alignment, and the physical properties of lasers, which may affect the outcomes of interference experiments.

greswd
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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?
 
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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?
 
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?
 
greswd said:
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?
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.
 
greswd said:
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?

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
 
greswd said:
same spot on a wall and see no light at all due to complete destructive
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.
 
DrClaude said:
Unless they are perfectly overlapping, you cannot get a perfect cancellation everywhere.
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.
 
You are right, there will always be an interference pattern.
 

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