# Why the term diffraction in XRD

1. Aug 30, 2010

### abhi2005singh

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"?

2. Aug 30, 2010

### Staff: Mentor

Re: Why the term "diffraction" in XRD

Diffraction on atoms is first, interference is second.

3. Aug 30, 2010

### Bob S

Re: Why the term "diffraction" in XRD

See discussion of Fresnel, Fraunhofer, and Bragg diffraction at

http://en.wikipedia.org/wiki/Diffraction

Bob S

4. Aug 30, 2010

### cmos

Re: Why the term "diffraction" in XRD

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.

5. Aug 30, 2010

### abhi2005singh

Re: Why the term "diffraction" in XRD

Thank you all for the help.