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Suppose I have $N$ slits separated by a distance $a$. We are sending a light source of wavelength comparable to the slit width. Then which of the effect should dominate: Interference or diffraction?

The doubt arose because, in the case of the 'X-ray diffraction in a solid-state, we are considering the diffraction of waves only. Why we are not concerned about the interference phenomena? Considering a linear chain of atoms separated by a distance $a$, which phenomena should dominate, and what is the condition for the dominance of interference or diffraction phenomena?

John Rennie
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walber97
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    Does this answer your question? What is the difference between diffraction and interference of light? – John Rennie Jul 12 '22 at 05:15
  • Among the answers, Feynman's lecture seems interesting to me. "When there are only a few sources, say two interference sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used". In a solid-state system, we have an infinite number of sources. Perhaps that's why in XRD we are considering the terminology Diffraction. – walber97 Jul 12 '22 at 05:28
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    There is no difference in the physics. In both cases we are adding vectors and the magnitude of the resultant depends on the directions of the vectors. It is purely a matter of terminology whether you choose to say interference or diffraction. – John Rennie Jul 12 '22 at 05:33
  • Yeah, I got it. Thank you! – walber97 Jul 12 '22 at 05:47

2 Answers2

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You are confusing different processes, interference is when multiple waves interacts between each other amplifying or canceling each other out. Now how these waves comes into interacting points, is a different process, i.e. beam splitter, thin-film interference, diffraction and others.

So at first something must "push" waves together for them to meet each other, and then they interacts in interference patterns. Classical example,- you bump pair of pebbles into lake creating two dot-sources of pressure waves, which when meets starts to interfere.

Agnius Vasiliauskas
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  • The process is the same. Diffraction is interference. In some cases the resulting field has a particular pattern such diffraction orders from a grating, the peaks in Young's experiment, and the shape of of the peaks from a single slit. But it's all a consequence of interference. – garyp Jul 12 '22 at 11:57
  • No, interference is not necessarily due to diffraction, it can be for example due to light reflection from different boundaries of some layer, check thin-film_interference. And I dare to hypothesize that maybe diffraction can be constructed without interference at all, maybe by guiding diffracted waves with waveguide apart or by other means. So all in all,- interference is wave superposition which can be (or can be not) summoned by diffraction or by other means. – Agnius Vasiliauskas Jul 12 '22 at 12:26
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So if you go back to Young's double slit experiment (YDSE) with slit width $a$ separated by distance $d$: you get interference. As $d\rightarrow 0$, you get diffraction from a slit of width $2a$.

What's the difference? Diffraction through a slit occurs because you don't know if the particle went through the left side of the slit or the right side. This doesn't seem to get noticed until the L and R sides have physical barrier between them.

If the two slits are one: it's diffraction, if you separate the slits: it's interference. If you add a bunch of slits: it's called a diffraction grating. Basically: the name doesn't matter, and the observed pattern is the Fourier transform of the aperture, regardless.

JEB
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