With respect to air,the refractive indices of ice and rock salt are $1.31$ and $1.54$ respectively. Calculate the refractive index of rock salt with respect to ice.

(1.18) The refractive index of a medium $2$ with respect to medium $1$ is given by the formula: ${}_{1}n_{2} = \frac{{}_{a}n_{2}}{{}_{a}n_{1}}$.
Here,medium $1$ is ice $(i)$ and medium $2$ is rock salt $(r)$.
Given: Refractive index of ice with respect to air $({}_{a}n_{i})$ = $1.31$.
Refractive index of rock salt with respect to air $({}_{a}n_{r})$ = $1.54$.
Therefore,the refractive index of rock salt with respect to ice $({}_{i}n_{r})$ = $\frac{1.54}{1.31} \approx 1.18$.