The equations consider a plane wave incident on a plane interface at angle of incidence θ θ i , a wave reflected at angle θ θ r = θ θ i , and a wave transmitted at angle θ θ t . In the case of an interface into an absorbing material (where n is complex) or total internal reflection, the angle of transmission does not generally evaluate to a real number. In that case, however, meaningful results can be obtained using formulations of these relationships in which trigonometric functions and geometric angles are avoided; the inhomogeneous waves launched into the second medium cannot be described using a single propagation angle.