ABSTRACT
The refractive index of a substance is the ratio of the speed of light in a vacuum to the speed of light in that substance. Several different methods have been used to measure and specify this well-known physical property of materials. Solubility is a measure of the amount of solute that is "absorbed" by a given amount of solvent. Hooke's law states that for small deformations the stress acting on a solid is proportional to the strain existing within it. The ratio of stress to strain is called the elastic modulus: the three common elastic moduli are Young's, sometimes called the stretch modulus; bulk modulus, which is the reciprocal of compressibility; and shear or rigidity modulus. For metals, the stress–strain curve shows an abrupt change at the elastic limit; for most optical materials the stress–strain curve gradually changes from a straight line to a curve with a decreasing slope.
