In Cartesian coordinates , let the region have refractive index and let the region have refractive index . Then the plane is the interface, and the axis is normal to the interface (Fig.11). Let and (in bold roman type) be the unit vectors in the and directions, respectively. Let the plane of incidence (containing the incident wave-normal and the normal to the interface) be the plane (the plane of the page), with the angle of incidence "θ" measured from towards . Let the angle of refraction, measured in the same sense, be "θ"t ("t" for "transmitted", reserving "r" for "reflected").