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Feynman said, "No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them."

I've noticed many cases here where people get thrashed, for example, for asking about the relationship between "diffraction and wavelength," because the person doing the thrashing is thinking actually of interference.

It's pretty simple. Diffraction is the spreading out or bending of light-waves upon traveling next to objects or exiting through openings (in the process of at least the latter of which the light-waves also become more spatially-coherent). While interference is the delivery or non-delivery of the light's energy at given points in space in connection with the light-waves' relative phases.

Even if that's not a perfect distinction, still, why such confusion? Is Feynman responsible for this abomination?

EDIT:

Examples of the Confusion here on Physics StacksExchange.

1) Question about diffraction and the relation between slit-width and wavelength.

Member with high reputation comments soon after question is posted, "You can clearly see diffraction (single slit diffraction) with a green laser pointer (so about 0.5 micron wavelength) through a 150 micron slit. That would seem to invalidate your question, unless a factor of 300x is not 'larger'." (This member is speaking of interference.) https://physics.stackexchange.com/questions/253532/diffraction-and-wavelength

2) Another question about diffraction and wavelength relationship.

Another member with high reputation comments soon after question is posted, "Wavelength doesn't affect diffraction at all, especially if you handle all your spatial values in units of wavelength." (This member is speaking of interference.) Diffraction wavelength relationship

Community
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David Reishi
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  • Duplicate twice over? http://physics.stackexchange.com/questions/137860/what-is-the-difference-between-diffraction-and-interference-of-light and http://physics.stackexchange.com/questions/79992/why-is-it-difficult-to-differentiate-between-interference-and-diffraction but it seems always to produce a good debate. – Farcher May 04 '16 at 07:25
  • I've deleted some off-topic comments. – David Z May 05 '16 at 09:39

2 Answers2

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These high reputation members aren't confused about the difference between diffraction and interference. The two terms are synonymous in the physics community (we're not arguing on the basis of linguistics. I can agree the terms are linguistically different just because people sometimes prefer one over another in certain contexts). The way the physics community uses the terms synonymously reflects the way that the math and physics is identical.

Do you agree that waves diffract more at small openings, especially when the opening is much smaller than the wavelength? That's because the distance between either side of the opening is too small for light to accumulate relative phase between the two points, allowing the light to propagate close to evenly in all directions. This changes once the opening approaches the size of a wavelength or larger. Every possible example of diffraction can be described in terms of interfering wavefronts.

Even looking at your definitions of diffraction and interference (which I think are quite reasonable for qualitative definitions), you can see similarities. Isn't the spreading of light according to your definition of diffraction also the delivery of light energy to different points in space, consistent with your definition of interference?

David
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  • "Isn't the spreading of light according to your definition of diffraction also the delivery of light energy to different points in space, consistent with your definition of interference?"..... No, not at all. Diffraction refers to the effect of light-waves bending or spreading out upon traveling close to an object or through an opening. Interference refers to the completely different effect of the light's energy being delivered versus not delivered at certain points based on the relative phases of the light-waves converging at those points. – David Reishi May 04 '16 at 18:04
  • @DavidReishi Can you clarify which parts of your definitions you think make diffraction inconsistent with interference? Do you agree that diffractional spreading changes the amount of energy delivered to different points in space? – David May 04 '16 at 20:06
  • "Can you clarify which parts of your definitions you think make diffraction inconsistent with interference?" ..... I'm not sure what you mean. If you point to any part of the definitions I gave, I'll try to clarify it. ..... "Do you agree that diffractional spreading changes the amount of energy delivered to different points in space?" ..... It changes the area in which energy may be delivered at any given distance, yes. For example, provided a single small slit, different wavelengths of light will diffract or spread out at different angles. – David Reishi May 04 '16 at 20:25
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As all the other commentators have said, both diffraction and interference are manifestations of the same thing: the fact that waves superpose.

The two words are slightly different, but it's clunky to work with two concepts when you only need one. You never do a "diffraction calculation" or an "interference calculation", you just do wave mechanics and superposition. I agree Feynman overreached a bit, but what he was getting at was that there was only one physical principle at play.

In any case, trying to standardize English words like this is hard. For example, under your definition, the pattern on the screen after a wave passes through a single slit is an example of interference. But most books would call it diffraction, i.e. it's an interference pattern that shows a diffraction effect. Other cases only get harder, i.e. what would you call the result of waves passing through two wide slits? It's enough of a mess that I think trying to introduce definitions (let alone calling people out for using the "wrong" definitions) is unproductive.

knzhou
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  • When a member asks about the relation between diffraction and wavelength, which is really basic, and someone with a high reputation immediately comments that "the question is all wrong because there is no relation between diffraction and wavelength!" (thinking all the while, not of diffraction, but of interference), it creates a really bad impression, not to mention that it's plain wrong. – David Reishi May 04 '16 at 17:55
  • @DavidReishi you are trying to separate a single phenomenon (wave propagation/diffraction/interference) into two different concepts. You cannot have wave amplitudes bending around corners without changing where the wave delivers energy. They are two different concepts, at face value, they are so intimately related that I think it causes more confusion to give them separate names. – anon01 May 04 '16 at 21:21