Find the speed of light in ice (n = 1.31) and in crown glass (n = 1.52).

The absolute refractive index is $n = \dfrac{c}{v}$, with $c = 3.00 \times 10^{8}\ \text{m/s}$. It has no unit and is never below $1$. A larger $n$ means slower light: optically denser. From material 1 into material 2, $n_{21} = \dfrac{n_2}{n_1} = \dfrac{\sin i}{\sin r}$.