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At angle of minimum deviation, we have angle of incidence equal to angle of emergence, that is, i = e.

Now, angle of incidence, emergence, prism angle and angle of minimum deviation are related as

D_m = i + e - A = 2i - A = 90 - A

Thus, D_m + A = 90°

 

Now, the refractive index of prism is given as;

 

 

Thus, angle of minimum deviation is

 

D_m = 90 - 60 = 30°