For water-air interface \(_w{\mu _a} = \frac{{\sin r}}{{\sin {{90}^o}}}\)…\((ii)\)

\( \Rightarrow \) \(_g{\mu _w} \times {\,_w}{\mu _a} = \frac{{\sin i}}{{\sin r}} \times \frac{{\sin r}}{{\sin {{90}^o}}} = \sin i\)

\( \Rightarrow \) \(\frac{{{\mu _w}}}{{{\mu _g}}} \times \frac{{{\mu _a}}}{{{\mu _w}}} = \sin i \Rightarrow {\mu _g} = \frac{1}{{\sin i}}\)