Types of Optically Active Crystals

Not every doubly-refracting crystal has just one optic axis. Crystals possessing only a single optic axis -- the special direction along which the ordinary and extraordinary rays travel at the same speed and no double refraction occurs -- are called uniaxial crystals; calcite, quartz, tourmaline and ice fall into this class, and it is these uniaxial crystals (specifically calcite) that the Nicol prism, discussed next, is built from. Crystals possessing two such optic axes instead are called biaxial crystals; mica, topaz, selenite and aragonite are common examples. The number of optic axes a crystal has is a fixed property of its own internal …