1.I am trying to understand two-source interference pattern. I am using this simulation for clarity.

As I understand, considering in classic-terms electromagnetic wave has both positive and negative magnitudes, i.e. its electric and magnetic components changes its values from some negative to some positive.

Due to superposition, negative and positive waves with same amplitude should cancel each other.

So, if consider em wave, as wave, that has negative values, as well as positive, we will have the next illustration:

enter image description here

In all interference (or diffraction) illustrations, explanations or simulations, (for example, like I mentioned above) I found, wave lines are colored same color, that is confusing me. Maybe they are considered wave as one-signed oscillation process?

However, if to consider waves as two-signed oscillation with positive and negative values, known two-source interference pattern seems to be partially wrong. How it should be:

enter image description here

From illustration above (if drew it correctly, maybe I missed something) we see, that, indeed, middle interference is constructive, but each second interference (every even interference) (in both direction from the middle one), should be destructive, because there occurs superposition of waves opposite magnitudes.

And on the screen, as I drew it in the right side of illustration, we should have at center higher value (sum of two waves), then in both direction, lower value (one wave) and not zero (!), then zero (sum of two opposite waves), then lower value (one wave) and all again. After wave half period - all the same, but with opposite sign.

Where do I wrong?

2.One more little question: do I understand correctly, that experiment with one source and two-split obstacle is more "popular" instead of just two monochromatic sources, because it is hard to make them coherent?

Well, I searched more images, that illustrates interference, like this, which seems to be correct, but at the same time this image illustrates each interference as constructive, as well as simulation, that mentioned at the beginning of this post.

So, it seems that they are either wrong, either consider waves as only with one sign, or has wrong scale between interference and detector pictures.

  • $\begingroup$ As a general comment: linear systems are "special" because they do not have self-interaction. Waves from different sources are simply passing through each other without changing their shape and without exchanging energy. So what we are calling "interference" is actually the absence of the scattering that is present in all non-linear systems. That is very easily overlooked while one is learning about waves in linear media for the first time. Thinking about interference as "non-interaction" is hard and it takes getting used to. $\endgroup$ Apr 3, 2023 at 14:47
  • $\begingroup$ "magnitude" is always positive, by definition, so when you say "waves of opposite magnitude", it's not correct. It's better to say "opposite phase (and equal magnitude)". The concept of "positive" and "negative" values is also coordinate dependent (that is: has no physical meaning), and again, it's better to consider phase. $\endgroup$
    – JEB
    Apr 3, 2023 at 15:25
  • $\begingroup$ @JEB, it is not my native language, but as far as I know, magnitude exactly means current value, while amplitude means maximum value, independently on sign (if symmetrical). Or something like that $\endgroup$ Apr 3, 2023 at 15:30
  • $\begingroup$ To truly understand "interference" of light you should consider Feynman and Dirac statements that explain that each photon does its own thing ... photons never cancel that's violation of conservation of energy. Photons take certain paths that have path lengths that are typically multiple of the wavelength ... see Feynman path integral on this site. Dark areas no energy/photons ... light areas get all the photons. $\endgroup$ Apr 3, 2023 at 19:15
  • $\begingroup$ @PhysicsDave, I think it would be better to start from classic consideration, and then quantum $\endgroup$ Apr 4, 2023 at 15:02

1 Answer 1


If you consider two coherent sources or two holes or slits does not matter. In your picture red is the pos max and blue the negative max, so you have constructive interference where two same colors meet and destructive when two different colors cross. At the point where two pos max cross you have a little T/2 later two negativ,the two oscillation at the points always add up to an oscillation with double amplitude. Where positive and negative meet it is always zero amplitude. What you write outside the picture is not clear. With 2) you are right, if the wave is light, for sound one uses two loudspeakers.

  • $\begingroup$ "...constructive interference where two same colors meet and negative when..." - did You mean destructive instead of negative? $\endgroup$ Apr 3, 2023 at 14:41

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