A ray of light is incident in medium 1 on a surface that separates medium 1 from medium 2. Let v1 and v2 represent the velocity of light in medium 1 and medium 2 respective. Also let n12 and n21 represent the refractive index of medium 1 with respect to medium 2 and refractive index of medium 2 with respect to medium 1, respectively. If i and r denote the angle of incidence and refraction, then: