since the ray undergoes minimum deviation, therefore, angle of emergence from second face, \(e=i=45^o\)

\(\therefore \quad \delta_{m}=i+e-A=45^o+45^o-60^o=30^o\)

\(\mu  = \frac{{\sin \left( {\frac{{A + {\delta _m}}}{2}} \right)}}{{\sin \left( {\frac{A}{2}} \right)}}\) \( = \frac{{\sin \left( {\frac{{{{60}^o} + {{30}^o}}}{2}} \right)}}{{\sin \left( {\frac{{{{60}^o}}}{2}} \right)}}{\text{ }}\)

\( = \frac{{\sin {{45}^o}}}{{\sin {{30}^o}}} = \frac{1}{{\sqrt 2 }} \times \frac{2}{1} = \sqrt 2 \)