The key to calcite’s behavior is its optic axis—a special direction, indicated by an arrow in Fig. 8.1, that runs through the crystal. Light polarized parallel to this axis travels through the crystal normally. Light polarized at right angles to the optic axis takes a deviant route—the extraordinary way. If the optic axis is oriented vertically, vertically polarized photons take the ordinary path (up), horizontally polarized photons go the extraordinary way (down).
