B What is Destructive Interference?

[sophiecentaur]

And you choose to ignore the evidence of your own eyes.

You need to try to look at this in a different way and interpret the evidence of your eyes - the theory isn't actually wrong. You have to want it to be right - unless you really think you have discovered something we have all missed.

there is always some trace of electromagnetism within a dark zone?

not necessarily; every experiment has imperfections but that is not why the 'zeros' are not exactly zero.

So light with zero amplitude has zero energy?

That statement is not wrong but the energy in one place will turn up elsewhere. As has been stated many times here, you cannot 'cancel' it everywhere.

How can it be said to exist if it has zero energy?

Why not? We say that you exist even though you are over there and not over here. Philosophically, the spaces between the words in this sentence are also parts of the sentence (which exists), even when there are no existing letters in them.

as if the wave had momentarily vanished.

That would imply that the wave doesn't exist at places where the E field happens to be at zero, as the wave moves along. You have to treat the wave as a whole entity and not just one point.
Take a much simpler example of waves than EM. Take a simple transverse standing wave on a string that's tied at each end. In the simplest case of a sinusoid, there are two waves, moving back and forth. If the string is not the right length, waves can still be traveling back and forth but you will get no interference because the phases of the two waves are 'marching past' each other and are not 0, 180, 270 or 360 at any particular places.

There is no displacement at the ends because the string can't move and neither is there displacement at any of the antinodes along the string. Does that worry you in the same way that a young's slits pattern? If energy is flowing, it doesn't have to imply that the displacement has to be non-zero everywhere. The principle behind it is Superposition because the two waves (ideally) do not interact.

 

[PeterDonis]

It would help if everyone just stayed on topic.

We are on topic. Off topic is not the issue here.

Telling me I’m wrong by giving me your definition of a wave is not proving anything.

We're telling you you're wrong because you have an incorrect understanding of what the EM field being zero at a point means.

I’m talking about one point in an EM wave.

Yes, we know. And we're telling you you have a wrong understanding of what the amplitude at one point being zero means.

In particular, in post #24 I quoted specific statements from your OP that were wrong and explained why they were wrong. Other people have told you similar things.

If anyone tries to bring in any additional concepts, I will ignore your post.

If you are going to ignore everything you think "brings in additional concepts", we might as well close this thread, since you have already put correct answers into that category. The issue is not that we need to stop posting off topic. The issue is that you need to start actually reading what we post and understanding what we are telling you about the wrong items in your understanding of the topic.

 

[weirdoguy]

Telling me I’m wrong by giving me your definition of a wave is not proving anything.

Our? It's the standard definition. If you define waves differently then tell us that, but be aware that your definition must be backed up by scientific literature. Ours is.

 

[Drakkith]

It would help if everyone just stayed on topic. Telling me I’m wrong by giving me your definition of a wave is not proving anything. I’m talking about one point in an EM wave. If anyone tries to bring in any additional concepts, I will ignore your post.

I've done just about everything I can think of to help you, but if you feel that most of what I've said is off topic and unrelated then I'm afraid I will have to bow out of this discussion. Have a pleasant day.

 

[Quarker]

Does anyone have an image of a good interference pattern they can post? I can’t do it with my phone.

 

[sophiecentaur]

Does anyone have an image of a good interference pattern they can post? I can’t do it with my phone.

Perhaps reading around would help you get what we're all telling you. There are images all over the place so do some work at it.

 

[weirdoguy]

You have one in post #2. Posting picture will not change anything, we've seen it hundreds of times.

 

[Dale]

Does anyone have an image of a good interference pattern they can post? I can’t do it with my phone.

How about the one posted by @Drakkith earlier

https://www.physicsforums.com/threads/what-is-destructive-interference.1010346/post-6576711

 

[Quarker]

not necessarily; every experiment has imperfections but that is not why the 'zeros' are not exactly zero.

What kind of imperfections exist within the dark zones of an interference pattern?

 

[Nugatory]

What kind of imperfections exist within the dark zones of an interference pattern?

There are many, but the ones that we expect to see in a practical double-slit experiment (by far the easiest way of generating an interference pattern with light - Thomas Young did it more than two centuries ago) will be caused by:
1) Our light is source is not perfectly monochromatic.
2) The edges of the slits are not exactly perfectly straight and the width of the slits will vary slightly across their length.
3) The surface of the screen we're projecting the pattern onto will not be perfectly flat; as you can see from the photo @Drakkith posted in post #2 a bump just one wavelength high will move that point on teh screen from a region of constructive interference to destructive, or vice versa.

Nonetheless, with reasonable care we can get a near-perfect interference pattern: for example https://physics.montana.edu/demonstrations/apparatus/6_optics/demos/doubleslitandlaser.html

 

[sophiecentaur]

What kind of imperfections exist within the dark zones of an interference pattern?

Have you read anywhere about interference of waves? I have a strong suspicion that you are trying to rely on asking PF questions and reacting to our answers. I can tell you with certainty that it is pointless to rely on Q and A to learn any subject. You have to read what has been actually written about the topic. Wiki always has stuff to say that's useful. As you read it, you need to answer your own questions from what you find in the text.
Answer this question:
Totally destructive interference will only work when there is total cancellation. Under what circumstances will two numbers, added together, produce zero? Then what is needed for two Fields to cancel each other out?

 

[Dale]

If you take a sinusoidal

What kind of imperfections exist within the dark zones of an interference pattern?

It is really not about any experimental imperfections. Even with perfect waves from perfect sources the derivatives are non-zero in the zone of complete destructive interference.

Specifically, if you have two opposed-phase coherent sinusoidal point sources separated in the x direction then the plane between those two sources has complete destructive interference. The value of the field is zero, but the value of the x-derivative is not.

 

[Ibix]

Worth noting that where the EM field is zero, I do not think that the potential is zero - or not necessarily. I think this is analogous to the nodes in a standing wave on a string being subject to a time varying stress.

 

[Charles Link]

It might interest the OP that by using a beamsplitter, such as in a Michelson interferometer, you can begin with two plane waves incident on the beamsplitter at right angles to each other, and the result can be all of the energy emerging in one direction or the other, depending on the relative phases. In this case, an entire region has destructive interference, and the other region is constructive.

 

[sophiecentaur]

It might interest the OP that by using a beamsplitter, such as in a Michelson interferometer, you can begin with two plane waves incident on the beamsplitter at right angles to each other, and the result can be all of the energy emerging in one direction or the other, depending on the relative phases. In this case, an entire region has destructive interference, and the other region is constructive.

Another great example* but the OP's problem goes far deeper than that. He appears to see a conflict between two aspects of waves and is doggedly avoiding a resolution.

*A simpler example of the same thing is two RF dipoles with appropriate spacing and phasing. Anyone could buy simple gear to achieve that out in the garden. M and M's experiment is a bit harder to implement.

 

[Quarker]

How about the one posted by @Drakkith earlier

https://www.physicsforums.com/threads/what-is-destructive-interference.1010346/post-6576711

Although very pretty, an image of waves on a pond is hardly analogous to an electromagnetic interference pattern.

 

[Dale]

Although very pretty, an image of waves on a pond is hardly analogous to an electromagnetic interference pattern.

You didn't ask for an electromagnetic interference pattern. You asked only for "a good interference pattern".

Regardless, the plain fact remains that the first derivative of the waves are non-zero in the region of complete destructive interference. So there is no sense that "the EM has momentarily left our universe" at those points.

 

[Drakkith]

Although very pretty, an image of waves on a pond is hardly analogous to an electromagnetic interference pattern.

You are simply incorrect. And what's worse, you are willfully incorrect.

@Dale requesting that this thread be locked as the OP obviously has no desire to learn anything new.

 

[Dale]

@Dale requesting that this thread be locked as the OP obviously has no desire to learn anything new

Request granted