The discovery of X－ray diffraction

The contributions of P.P. Ewald, M. von Laue, W.H. Bragg (father) and W.L. Bragg (son) to the discovery of X-ray diffraction in 1912 are discussed. While discussing Ewald‘s Ph.D. thesis supervised by Sommelfeld in Munich, Laue learnt of the translation periodicity in a crystal lattice, which was about the same order of magnitude as the wavelength of X－rays. This gave him the idea that X－ray diffraction in a crystal could be treated by adding one more diffraction equation to the two－dimensional diffraction of a cross－grid pattern. Under the guidance of this hypothesis, Friedrich and Knipping carried out X－ray diffraction experiments in April 1912, first on orthorhombic CuSO4 and later on cubic ZnS, and soon found definite evidence of X－ray diffraction. This proved not only the wave nature of X－rays but also the three－dimensional periodicity of a crystal. After receiving this news in the summer of 1912 in Leeds, W.H. Bragg tried to explain it by the particle theory of X－rays, while his son W.L. Bragg carried out similar diffraction experiments in Cambridge. Based upon the elliptical shape of the diffraction spots and the crystal lattice theory learnt from Pope and Barlow, W.L. Bragg came to the conclusion that X－ray diffraction in a crystal could be treated as reflection from parallel lattice planes, and hence derived the famous Bragg equation. The Braggs then developed a new branch of science, namely the crystal structure analysis of simple inorganic compounds, minerals, organic molecules, and biological macromolecules. Both Laue and the Braggs acknowledge the importance of the very favorable environment in Munich and Cambridge that led to the discovery of X－ray diffraction. In view of the important contributions of Ewald to the discovery of X－ray diffraction, his reciprocal lattice theory and later his dynamical diffraction theory, many crystallographers consider that he should also have received the Nobel prize.