The discovery of X-ray diffraction

Starting from the discovery of Roentgen’s X-ray (Ro¨ntgen 1895), I here reconstruct the various attempts carried out to find X-ray diffraction using very thin wedge-shaped slits. These experiments allowed A. Sommerfeld to achieve, at the beginning of 1912, a qualitative estimate of the wavelength of X-rays around 4 9 10-9 cm. This value was at the base, together with the adoption of the space-lattice model for the crystal, of Laue idea of using a crystal as a diffraction grating for X-ray. Then we will discuss the feasibility of the Laue project, as well as the realization of the experiment, and the first results of W. Friedrich and P. Knipping, with the theoretical interpretation given to them by Laue. Laue’s discovery of X-ray diffraction was immediately taken up by William Henry Bragg and re-interpreted by his sonWilliam Lawrence in terms of X-ray reflection by the planes of the crystal, thus becoming, in a few months, a formidable tool for the study of crystal structures, as well as for the precise determination of the wavelengths of X-rays (to Bragg father we owe the construction of the first X-ray ionization spectrometer). Particular attention will be given to the works of C. G. Barkla on X-ray absorption, which resulted in the discovery in 1906 of the Roentgen characteristic radiation of the elements. These works strengthened the hypothesis of the wave nature of X-rays, at the expense of the ‘‘corpuscular hypothesis’’ (supported notably by J. Stark and by W.H. Bragg), and represented a solid reference point for the project of Laue.