Why the term diffraction in XRD

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Discussion Overview

The discussion centers around the terminology used in X-ray diffraction (XRD), specifically questioning why the term "diffraction" is applied when discussing the selective reflection of X-rays from crystals. Participants explore the definitions and implications of diffraction versus interference in this context.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the appropriateness of the term "diffraction," suggesting it may be a misnomer and proposing that the phenomenon could be more accurately described as "interference."
  • Another participant asserts that diffraction occurs at the atomic level before interference, implying a sequence in the phenomena.
  • A participant references the historical context of diffraction, mentioning Fresnel, Fraunhofer, and Bragg diffraction, suggesting that understanding these concepts may clarify the terminology.
  • One participant challenges the assumption of specular reflection from atomic planes, arguing that the wavelengths of X-rays are comparable to atomic spacings, complicating the notion of reflection.
  • This same participant emphasizes that a proper derivation of Bragg's law involves Fourier analysis in reciprocal space, leading to a definition of diffraction that does not rely solely on reflection or refraction.
  • It is noted that interference and diffraction are not entirely separate concepts, with examples provided where both explanations can apply, such as in the case of oil slicks and Young's double slit experiment.
  • Some XRD specialists from materials engineering backgrounds refer to "x-ray reflectometry" as synonymous with XRD, indicating a potential overlap in terminology.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of the term "diffraction" in the context of XRD, with no consensus reached on whether it is a misnomer or an accurate descriptor. The discussion remains unresolved regarding the definitions and implications of diffraction versus interference.

Contextual Notes

The discussion highlights the complexity of defining diffraction and interference, with participants noting assumptions about atomic reflection and the mathematical derivations involved. There are also references to specific historical figures and concepts that may influence the understanding of these terms.

abhi2005singh
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Why the term "diffraction" in XRD

When there is selective reflection of x-rays from crystals, why the phenomena is called "diffraction"? As we know that diffraction is associated with the bending of waves at the corners of any obstacle, here there does not seem to be any bending around the corner. So why the name diffraction? Since it is an interference between the waves emitted from the upper and lower plane of atoms, shouldn't it be called "interference"?

Some possible answers can be the following:
1. It is an unfortunate misnomer. (One of my colleagues told me this recently)
2. The idea of diffraction stated above is not correct or we can say it is not accurate. Then how to define "diffraction"?
 
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Diffraction on atoms is first, interference is second.
 


There's two answers here (but I'm sure there may be others):

First of all, the assumption of specular reflection from "planes of atoms" is a very big assumption to make. Indeed, with XRD wavelengths being on the order of (and usually smaller than) the atomic spacing, how can one ever assume specular reflection?! The only reason the Braggs' simple explanation was so convincing is that it gave the correct results.

(Still on the first answer) Now a proper derivation of Braggs' law requires a Fourier analysis on a reciprocal space of the crystal lattice (i.e. the reciprocal lattice). The end result is a non-rectilinear deviation of a ray of light that is neither due to reflection nor refraction. Such is the definition of diffraction.

The second answer (somewhat of a more general nature): Interference and diffraction are not entirely separate entities. It is true that there are cases that can only be explained through interference or through diffraction, but there are cases where either explanation does suffice.

Examples:
1) The appearance of colors on a watery oil slick is properly explained by interference.
2) The inability to focus light to an infinitesimal spot is properly explained by diffraction.
3) Young's double slit experiment is properly a problem of diffraction, however, it can also be explained as the interference of secondary Huygens waves.

It is interesting to note, however, that lots of XRD specialist (at least coming from a materials engineering background) tend to talk about "x-ray reflectometry" as a synonym for XRD.
 


Thank you all for the help.
 

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