The relation between angle of deviation and angle of incidence, angle of emergence and angle of prism is given by
Angle of deviation = i1 + i2 – A
For maximum deviation, Angle of incidence (i1) = 90°
Angle of emergence (i2) = 90°
We know, μ = 1/ sin C . 1/ sin A/2 [Where C = Critical angle, A = Angle of prism]
∴ A = 2C
∴ Maximum deviation = i1 + i2 – A = 90 + 90 – 2C = 180 – 2C = n – 2C.