Let i be the angle of incidence, e be the angle of emergence, A be the refracting angle of prism andδbe the deviation produced. In a prism,

i + e - A =δ

=> i + e = A +δ

The condition when the angle of deviation between the incident ray and emergent ray is the least for a ray passing through a prism is considered as minimum deviation in a prism.

In the condition of minimum deviation,

angle of incidence (i) = angle of emergence (e)

angle of refraction at face 1 (r1) = angle of refraction at face 2 (r2)

angle of deviation = angle of minimum deviation