A ray of light is incident at the glass–water interface at an angle i, it emerges finally parallel to the surface of water, then the value of µ_g would be: 

For glass water interface  ${ }_g{\mu }_{\omega }=\frac{\sin i}{\sin r} ......\left(i\right)$

For water-air interface ${ }_{\omega }{\mu }_a=\frac{\sin r}{\sin 90} ......\left(ii\right)$

${ }_g{\mu }_{\omega }{\times }_{\omega }{\mu }_a = \sin i \Rightarrow {\mu }_g =\frac{1}{\sin i}$