Michael Glazer, Visiting Professor at the University of Warwick

Published August 2013

What are the burning questions in physics? Just over 100 years ago, following the discovery of X-rays by Wilhelm Röntgen, one of physics’ burning questions related to the nature of Röntgen’s mysterious rays. Did they consist of particles or did they consist of waves?

In 1912, while working in Münich, Max Laue suggested that if one could diffract X-rays, rather like is done by light passing through narrow slits, then this would show that X-rays were in fact wave-like. The problem was where could one find such a narrow set of 'slits' to cause the diffraction? It was known that if X-rays were indeed waves, their wavelengths would be exceedingly small, much less than for light? In a brilliant flash of inspiration he realised that crystals, which were thought to consist of repeating patterns of atoms and molecules, might act like a fine three-dimensional grating capable of diffracting X-rays.

Despite local opposition at the time, a successful experiment was carried out by Laue’s colleagues, Friedrich and Knipping in April 1912, in which X-rays incident on crystals of copper sulphate, zinc sulphide and diamond gave rise to sharp spots on a photographic plate. This seemed to indicate that the X-rays were diffracted by crystals, and therefore were wave-like. Laue obtained the Nobel Prize for Physics in 1914 for this work. However, Laue had problems in explaining the pattern of spots.

Laue’s experiments were brought to the attention of William Henry Bragg (William) in the summer of 1912 when the Norwegian physicist and physical chemist Lars Vegard wrote to William. An adherent to the particle theory of X-rays, William set about repeating Laue’s experiments together with his 22-year old son William Lawrence Bragg (Lawrence) with a view to showing that the patterns of spots could be explained by particles of X-rays passing down channels in the crystals. The experiments failed but, in November 1912, Lawrence had the idea that if one considered the X-rays to be waves, and treated them as if they were reflected from parallel planes of molecules in the crystal, he could very simply explain Laue’s patterns. Lawrence developed a simple formula (below) to explain the positions of all the spots, which he called ‘reflections’ by analogy with reflections from mirrors.

nλ = 2dsinθ

λ is the X-ray wavelength, d the spacing distance between a set of planes, and θ is the angle the X-ray beam makes with the planes. This is Bragg’s Law and is one of the most famous equations used across many areas of science.

Lawrence showed that the pattern of spots could be used to determine the arrangement of molecules in a crystal of zinc sulphide. From this came the new science of X-ray crystallography in which, by studying the intensity of the reflections, one can in principle determine the positions of all the atoms in a crystal – what we call crystal structure determination.

The field of crystallography is relatively unknown among the general public, with the names of the two Braggs largely unrecognised. This may be because the subject is interdisciplinary; crystallographers can be found in departments of physics, chemistry, materials, life sciences, engineering and, amongst other areas, mathematics. The results of crystallographic research are used to underpin most of the science of solid materials, and in terms of wider public recognition, the story of the Braggs, and the science they founded, has tended to remain unknown.

In early 1913, Lawrence worked on diffraction by crystals of common salt (NaCl) using Laue’s method together with a new type of spectrometer, the ionization spectrometer. This device was created by William H Bragg who, by this time, had also come to the view that X-rays could be treated as waves (William is quoted as having said that “physicists use the wave theory on Mondays, Wednesdays and Fridays and the particle theory on Tuesdays, Thursdays and Saturdays." Whilst, presumably, in jest, William was ahead of his time. L. de Broglie later showed that electromagnetic radiation could be treated either as waves or particles depending on the experiment being used).

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Professor Michael Glazer is a Visiting Professor at the University of Warwick and Emeritus Professor of Physics at the University of Oxford. Mike’s research interests are in the relationship between crystal structure and physical properties; crystallographic instrumentation. His hobby is flying; he owns a 2005 Cirrus SR22, which he flies everywhere, especially to and from France.

Image: Braggs images c/o The Royal Institution of Great Britain.