A student sends a ray from air into a block of plastic.

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, $n_{21} = \dfrac{n_2}{n_1} = \dfrac{\sin i}{\sin r}$.