The refractive index of crown glass is $1.52$. What is the meaning of this statement?

1. The refractive index of medium 1 with respect to medium 2 ${(}^2{\mu }_1)$ is given by the ratio of the speed of light in medium 2$\left(v_2\right)$ to the speed of light in medium 1$\left(v_1\right)$.

Mathematically, ${}^2{\mu }_1=\frac{v_2}{v_1}$

2. The refractive index of crown glass with respect to vacuum is 1.52.

So, $1.52=\frac{v_v}{v_c}$, where $v_v$ is the speed of light in a vacuum, and $v_c$ is the speed of light in crown glass.

$v_v=1.52v_c$

3. It means that light travels 1.52 times faster in a vacuum than it does in a crown glass.