The classical treatment of x-ray diffraction phenomena

The earliest experiments on the diffraction of X-rays by crystals showed that the directions in which the rays were diffracted were governed by the classical laws of optics. Laue's original paper on the diffraction of white radiation by a crystal, and the work which my father and I initiated on the reflection of lines in the X-ray spectrum, were alike based on the laws of optics which hold for the diffraction grating. The high accuracy which has been developed by Siegbahn and others in the realm of X-ray spectroscopy is the best evidence of the truth of these laws. Advance in accuracy has shown the necessity of taking into account the very small refraction of X-rays by the crystal, but this refraction is also determined by the classical laws and provides no exception to the above statement.

The first attempts at crystal analysis showed further that the strength of the diffracted beam was related to the structure of the crystal in a way to be expected by the optical analogy. This has been the basis of most work on the analysis of crystal structure. When monochromatic X-rays are reflected from a set of crystal planes, the orders of reflection are strong, weak, or absent in a way which can be accounted for qualitatively by the arrangement of atoms parallel to these planes.