ABSTRACT
This chapter describes crystals, glass ceramics, and two-phase glasses which are important to the art and science of optical devices. The crystalline state is characterized by position regularity of the atoms constituting the solid. Covalent crystals have high hardness, are refractory with high melting points, and when pure, have low electrical conductivity at low temperature. The optical properties of crystals are different for various orientations of the crystal with respect to an incoming light ray. However, crystals with cubic symmetry exhibit no birefringence, while noncubic crystals exhibit birefringence effects. Many natural crystals have been found and an excellent collection resides in the Museum of Natural History in New York City. Crystalline materials with more complex chemistries exist in many binary, ternary, and higher order phase equilibrium systems. Diamond is an exotic crystalline material. Diamond is a cubic crystal and has no intrinsic birefringence. However, internal strains are manifested as birefringence.
