Optical Analogue of X-Ray and Electron Diffraction Patterns ;

Abstract

IN a recent letter by J. Dyson concerning an optical analogue of X-ray and electron diffraction patterns¹, no reference is made to the previous experiments made with other models for exhibiting such analogies. A particularly complete demonstration of this type was published by Y. V. Kathavate² working at Bangalore. Kathavate used small circular copper disks having a uniform size of about 0·6 mm. as scattering centres. These were punched out of thin copper foil using a special technique so that the edges were free from irregularities. The disks were distributed on a glass plate, and by suitable manipulation, such as bowing the edge of the plate, the disks could be made to take up any desired distribution. The Fraunhofer diffraction patterns produced by these were then photographed both in monochromatic and white light and were reproduced in four plates accompanying Kathavate‘s paper. They exhibit a whole range of phenomena—starting from the analogy of a gas in which the molecules are far apart giving rise to the typical corona with a central maximum of intensity, passing through the liquid stage giving a halo with a central minimum, then through a randomly oriented and a preferredly oriented poly-crystal which both give spotty rings, and ending up with the analogy of a perfect crystal which gives a hexagonal array of spots.