Snell's law gives the relation between the angle of incidence and the angle of refraction when a beam of light moves from one medium to another. If the angle of incidence is Ai and the angle of refraction is Ar, the two are related by sin Ai / sin Ar = V1 / V2, where V1 and V2 is the velocity of light in the first and second medium respectively.

The law does not give correct values in case there is total internal reflection. Total internal reflection occurs when the ratio of the speed of light in the second medium to that in the first medium is greater than 1. For example if the ratio of the velocities V2/ V1 is given by 1.5 and the angle of incidence is 60 degree, we have sin Af = 1.5 * 0.8660 = 1.299 which is not possible as the value of the sine of an angle cannot exceed 1.