c
(c) \(_D{\mu _R} = \frac{{\sin i}}{{\sin r'}}{ \Rightarrow _R}{\mu _D} = \frac{{\sin r'}}{{\sin \;i}} = \frac{1}{{\sin C}}\)

\( \Rightarrow \sin C = \frac{{\sin i}}{{\sin (90 - r)}} = \frac{{\sin i}}{{\cos r}} = \frac{{\sin i}}{{\cos i}}\) (as \(\angle i = \angle r\)) 

\(\Rightarrow \sin C = \tan i \Rightarrow C = {\sin ^{ - 1}}(\tan i)\)