In Cartesian coordinates (x, y, ‍ z), let the region ‍ y < 0 ‍ have refractive index n 1 ‍, ‍ and let the region ‍ y > 0 ‍ have refractive index n 2. Then the xz plane is the interface, and the y axis is normal to the interface (Fig. 11). Let i and j (in bold roman type) be the unit vectors in the x and y directions, respectively. Let the plane of incidence (containing the incident wave-normal and the normal to the interface) be the xy plane (the plane of the page), with the angle of incidence θ i measured from j towards i. Let the angle of refraction, measured in the same sense, be θ t (t for transmitted, reserving r for reflected).