8 X - ray diffraction : the contributions of Max von Laue , W . H . and W . L . Bragg and

The experimental technique which has been of the greatest importance in revealing the structure of crystals is undoubtedly X-ray diffraction. The story of the discovery of X-ray diffraction in crystals by Laue,∗ Friedrich and Knipping in Munich in 1912 and the development of the technique by W. H. Bragg∗ andW. L. Bragg∗ in Leeds and Cambridge in the years preceding the First World War is well known. But why did the Braggs make such rapid advances in the analysis of X-ray diffraction photographs in comparison with Laue and his co-workers?An important factor in the answer seems to be that Laue envisaged crystals in terms of a three-dimensional network of rows of atoms and based his analysis on the notion that the crystal behaved, in effect, as a three-dimensional diffraction grating. This approach is not wrong, but it is in practice rather clumsy or protracted. On the other hand, the Braggs (and here the credit must go to W. L. Bragg, the son) envisaged crystals in terms of layers or planes of atoms which behaved in effect as reflecting planes (for which the angle of incidence equals the angle of reflection), strong ‘reflected’ beams being produced when the path differences between reflections from successive planes in a family is equal to whole number of wavelengths. This approach is not correct in a physical sense—planes of atoms do not reflect X-rays as such—but it is correct in a geometrical sense and provides us with the beautifully simple expression for the analysis of crystal structure: