Given, The refractive index of air with respect to glass is $\frac{-2}{3}$ Refractive index of water with respect to air $=\frac{4}{3}$ Speed of light in glass $=2\times10^8\text{m/s}$ Refractive index of air with respect to glass $\text{n}_\text{ag}=\frac{\text{V}_\text{g}}{\text{V}_\text{a}}$$\frac{\text{V}_\text{g}}{\text{V}_\text{a}}=\frac{2}{3}$