In gemology and optical mineralogy, total reflection occurs in a transparent solid where a light ray strikes the surface of a medium of lower refractive index at any angle greater than its critical angle as defined sin r = 1/n, where r is the critical angle and n is the refractive index of the solid (or n₂/n₁, where n₁represents the lower refractive index if other than 1 for air). Compare: critical angle; reflection; law of refraction.