total (internal) reflection A beam of light traveling from denser to rarer medium deflects away from normal at the boundary of the two mediums. The angle of incidence for which the angle of refraction is 90^o is called critical angle q _c.

q _c = sin^-1[m ₁ / m ₂ ] (t6)

where m ₁ and m ₂ are the refractive indices of rarer and denser medium respectively. If the angle of incidence is larger than the critical angle then the light beam is totally reflected. Totally reflecting prisms are used in many optical instruments instead of mirrors (see fig.t6 and fig. t7).