Each angle of incidence at P, mark point Q where light ray leaves block. Angle of refraction if the angle between normal at P and line PQ. Measurements of angles of incidence and refraction for different incident rays show that: angle of refraction, r, at P is always less than angle of incidence, i, and the ratio of sini/sinr is the same for each light ray. This is known as Snell's law. Ratio referred to as the refractive index, n.