The alternative method of treating the results of the last experiment suggests a way of finding the path of a ray through a glass block or some other medium, when i and n are given.

Example. Trace the path of a ray through a glass block of refractive index 1.52, for light entering at an angle of incidence of 45°. Measure the angle of refraction.

Draw SS’ to represent the surface of separation between glass and air and AO an incident ray making an angle of 45° with the normal NO (Fig. 23.5). Then, using the largest possible scale for best accuracy, mark off OD equal to 1.52 units of length. At D erect a perpendicular to cut AO at B. With centre 0 and radius OB, draw a circle. Along OS’, mark off OE equal to I unit of length. Through E draw a perpendicular to SS’ to cut the circle at C. Then OC is the required refracted ray and the angle of refraction r is found to be 28°.