Refraction

In addition to sifting and sorting and mixing colors or light wavelengths, solid crystal structures can perform other marvelous operations with light. They are even capable of bending it or, perhaps more correctly, changing its direction. All lenses, whether for telescopes or eyeglasses, are designed to take advantage of this fact. By changing the direction of travel of a light image, a lens deceives the eye and brain into giving the impression that the image is coming from a fictitious direction. Light traveling from one substance to another will be bent varying amounts, depending on the densities of the substances involved. The greater the difference in density the greater the change of direction. Many mineral species can bend or refract light in two directions at the same time. The light beam is actually split into two parts by each part being bent a different amount. Refraction and double refraction are caused by the way a mineral's atoms affect light. Each mineral, then, has its own kind and amount of refraction or double refraction which can be measured and used for identification purposes.