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See diagram.  we are looking at refraction from lighter medium to a denser medium.

Critical angle = c = angle of incidence = i
Refractive index = μ = Sin i / Sin r   and    r = 90°  for critical angle of incidence

Minimum deviation is when the angle of incidence is = 0. and angle of refraction is also 0.  That is the ray is incident perpendicular to the boundary.

As  angle of incidence increases from 0, the angle of refraction increases faster than angle of incidence.  So the deviation increases.  Thus the maximum deviation occurs when the critical angle is approached.
                   Deviation = 90° - c

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