Reflexion of X-rays with change of frequency

Abstract

In Current Science of April, 1940, photographs were published exhibiting the very remarkable phenomena observed when a pencil of X-radiation from a copper target traverses a cleavage plate of diamond and the resulting Laue diagrams are recorded with adequate exposures. The photographs showed monochromatic X-ray reflexions from the (111) planes of the crystal which were quite sharply defined at all settings of the crystal but which were evidently different from the well-known Bragg reflexions. As this phenomenon was inexplicable on the existing Triple quantum reflexions by the (111) planes in diamond. X-ray theories, it was interpreted as follows: X-ray photons traversing a crystal can exchange energy with it, giving up or taking on a quantum of energy corresponding to the frequency of one or another of the infra-red or high-frequency vibrations of the crystal lattice ; the X-rays are then geometrically reflected by the lattice planes of the crystal with a change of frequency. Further study of the effects exhibited by the (111) planes of diamond showed them to be in complete accord with the ideas outlined in the article of April, 1940 (see accompanying illustration). If the fundamental process stated is possible with diamond, it should also occur and be observable with other crystals. Investigations were made which showed that this is actually the case ; these are described in a whole series of papers published in the Proceedings of the Indian Academy of Sciences and in Current Science from May, 1940, onwards.