The smallest angle at which a wave passing from a substance of higher refractive index to one of lower refractive index can be incident on the surface between the substances and be totally reflected at that surface. At the critical angle, some of the waves may propagate along the surface. At angles less than critical, some or all of electromagnetic waves propagate into the surface. The angle is expressed by the formula sin(critical angle) = N'/N, in which N is the higher refractive index and N' is the lower.