Impact Figures on Polished Rock Salt Surfaces

Abstract

IF a small steel ball is dropped from a height of a few inches on a polished rock salt surface, the imprint of the ball on the crystal surface remains as a circular depression of one or two millimetres in diameter. The surface is deformed, however, over a region many times the area of the circular depression. If an optical test plane (a piece of ordinary plate-glass will do) is placed on the crystal and the surface examined in monochromatic light, the interference pattern shows a number of families of ‘loops’ extending away from the imprint of the ball.