This chapter introduces the principal axes and principal refractive indices of a crystal, and addresses these questions: For light propagating in an anisotropic crystal, if the direction of the k vector is known with respect to the principal axes,
1. What are the directions along which the electric field vector E and the displacement vector D oscillate?
2. What is the angle between the k vector and the Poynting vector S?
3. What is the propagation equation describing the spatial change of the electric field amplitude for the propagation of an electromagnetic wave through an anisotropic medium?
Propagation in a biaxial crystal in an arbitrary direction will be considered first. Then, special directions ...