#bragg diffraction

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Aurora Opal (Macro Photography)

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Opalized wood Author: Peter C. Huber, Wien / Austria

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Hazelnuts - A new source for opals

[qmulmri-blog]

One hundred years of diffraction: "probably the most important discovery you have never heard of"

An interesting piece about the centenary celebration of the development of x-ray crystallography by the father and son team William and Lawrence Bragg published yesterday in the Metro.

Essentially 100 years ago the Braggs reported their understanding of what happens when a beam of x-rays – electromagnetic radiation like light and radio waves, but with a wavelength as small as the size of an atom – strikes and is reflected by a crystal (the original paper can be downloaded from this link). A crystal is a regular array of atoms, and viewed from different directions give the impression of there being layers of atoms. What the Braggs worked out was how the x-rays can reflect from these individual layers, and they gave a mathematic equation that links the angle of reflection to the spacing between layers in that particular view. So if you know the angles that the x-rays are reflected though, you can reconstruct the fundamental structure of the layers, namely what crystallographers call the crystal lattice.

The article has a rather nice graphic that I reproduce at the end. The point of the article is to say that 100 years ago there was no method to look at materials with a microscope that takes you down to the size of the atom, and the significance of what the Braggs did was to make that possible, both from a theory point of view but also from a practical point of view. The field of crystallography owes a debt to many more people, as seen by the fact that several Nobel prizes have been won by crystallographers, and converting the intensity of the reflected beams into information about the electron density and hence the atomic structure came from a number of works and is somewhat more complicated than the article suggests, but that would be too much for one newspaper article.

The nice graphic is this: