Water and glass have absolute refractive indices of 4/3 and 3/2 respectively. What will be the refractive index of glass with respect to water?

The absolute refractive index of a media is the ratio of the speed of light in a vacuum to the speed of light in that medium.

$\mathrm{\mu }=\frac{\mathrm{Speed} \mathrm{of} \mathrm{light} \mathrm{in} \mathrm{vacuum}\left(\mathrm{C}\right)}{\mathrm{Speed} \mathrm{of} \mathrm{light} \mathrm{in} \mathrm{medium} \left({\mathrm{v}}_{\mathrm{medium}}\right)}$

The absolute refractive index of water, ${\mathrm{\mu }}_{\mathrm{w}} =4/3$

The absolute refractive index of glass, ${\mathrm{\mu }}_{\mathrm{g}} =3/2$

Refractive index of glass with respect to water $=\frac{\mathrm{Speed} \mathrm{of} \mathrm{light} \mathrm{in} \mathrm{water} }{\mathrm{Speed} \mathrm{of} \mathrm{light} \mathrm{in} \mathrm{glass}}$

Since the speed of light in a medium is inversely proportional to its absolute refractive index, 

Refractive index of glass with respect to water $=\frac{{\mathrm{\mu }}_{\mathrm{g}}}{{\mathrm{\mu }}_{\mathrm{w}}}$

$= \frac{3/2}{4/3}= \frac{3 \times 3}{2\times 4}= \frac{9}{8}$

Therefore, the refractive index of glass with respect to water will be 9/8.