The work of Laue and Ewald

Laue's was not the first attempt to detect diffraction phenomena of X-rays from gratings and slits. Indeed, the failure to do so provided one argument for the corpuscular hypothesis. Laue's great achievement rested, as it were, on two 'ifs'. 'If' X-rays were waves they might be expected to have very small wavelengths and 'if' crystals consisted of regular arrangements of atoms, the spacing of the atoms and the wavelength of the X-rays might be comparable - in which case one would expect that crystals would diffract X-rays in the same way that finely ruled gratings diffract light.

But this insight did not come 'out of the blue', but from his reading, and discussion in January 1912, of the PhD thesis of a young research student at Munich, Paul Peter Ewald. Indeed, Ewald's thesis contained perhaps the most fertile ideas in the whole subject of diffraction (the reciprocal lattice and sphere of reflection) and if history had been just he should also have been awarded a Nobel Prize.

Laue (who was 'only' the equivalent of a lecturer) took his idea to Arnold Sommerfeld, Professor of Theoretical Physics at Munich and who said, essentially, that it wouldn't work on account of the thermal vibrations of the atoms. (Sommerfeld was partly right, thermal vibration does give rise to a decrease in intensity of the reflections).

However, Laue persevered, and with the help of two young research workers, Walter Friedrich and Paul Knipping, the first diffraction photographs (albeit poor and smudgy) of a copper sulphate pentahydrite crystal were first made on April 12 1912. This crystal was quickly replaced by carefully prepared crystals of zinc blende (ZnS-sphalerite) which has a much simpler crystal structure, the symmetry of which was revealed from the clear pattern of diffraction spots.