Properties Depending Upon Light and Refraction of Light in Minerals

Refraction of Light in Minerals
When light comes into contact with a transparent mineral, part of it is reflected from the surface of the mineral and part enters the mineral. The light which enters the mineral is in general refracted. When light passes from a rarer into a denser medium, as in the case of passing from air into a mineral, its velocity is retarded. This change in velocity is accompanied bye a corresponding change in the direction in which the light travels, and it is this change in direction of propagation that is known as refraction of light. The amount of refraction of a given light ray is directly proportional to the ratio existing between the velocity of light in air and in the mineral. The ratio between these two velocities is known as the index of refraction, or n, of a mineral is 2, light will travel in it with one-half the velocity it has in air.

The refractive power toward light which a mineral possesses has often a distinct effect upon the appearance of the mineral. For example, a mass of cryolite may almost always be told at sight, though, as is generally the case, there is no crystal shape to aid in the identification. The mass has a peculiar appearance, something like that of wet snow, and quite different from that of ordinary white substances; and this is due to the fact that the index of refraction of cryolite is unusually low for a mineral. An instructive experiment may be tried by finely pulverizing some pure white cryolite and throwing the powder into water, when it will apparently disappear, as if it had instantly gone into solution. The powder, however, is insoluble and may be seen indistinctly as it settles to the bottom of the vessel. The reason for this disappearance of the cryolite is that its index of refraction (about 1.34) is near that of water (1.335), hence the light travels almost as readily through the mineral as through water, and consequently it undergoes little reflection or refraction.

Substances having an unusually high index of refraction have an appearance which it is hard to define, and which is generally spoken of as adamantine luster. This kind of luster may be comprehended best by examining specimens of luster may be comprehended best by examining specimens of diamond (n = 2.419) or of cerussite (n = about 3.2). They have a flash and quality, some diamonds almost a steel-like appearance, which is not possessed by minerals of low index of refraction; compare, for example, cerussite and fluorite (n = 1.434). It is their high index of refraction that gives to many gem minerals their great brilliancy and charm.

In the majority of cases the index of refraction of a mineral is not far from 1.5, and gives to minerals a luster which is designated as vitreous. Quartz (n = 1.55), feldspar (n = 1.52) and calcite (n = 1.57) are good examples.