Question about light waves canceling each other

AI Thread Summary
When two coherent laser beams traveling in the same direction are 180° out of phase, they can theoretically cancel each other out, leading to total destructive interference. This phenomenon does not violate the principle of energy conservation, as energy is redistributed rather than lost; constructive interference occurs elsewhere. Practical experiments, such as splitting a laser beam and recombining it after passing through different media, demonstrate that phase differences can create interference patterns. The discussion also touches on sound waves, suggesting similar behavior in pressure waves. Overall, the interaction of light and sound waves under these conditions adheres to classical physics principles without necessitating quantum explanations.
bouscher
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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! :)
 
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bouscher said:
What would happen when two identical yet 180° phase different pressure waves emanate from the same source and travel in the same direction.
Easy. This is equivalent to the source not emanating radiation at all.
 
DrDu said:
Easy. This is equivalent to the source not emanating radiation at all.

That's exactly what I thought, but how do you resolve the issue with light waves?
 
Which issue?
 
The issue is that theoreticly, one could create such a situation with light waves
 
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.
 
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.
 
Vanadium 50 said:
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.

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.
 
bouscher said:
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! :)

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

https://www.physicsforums.com/showthread.php?t=306962

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

Zz.
 
  • #10
bouscher said:
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

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.
 
  • #11
bouscher said:
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).
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:

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

bouscher said:
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.
The situation you are describing is completely classical. No need to go into quantum stuff.
 
  • #13
Thanks people :) I'll be sure to look into those links...
 

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