Refractive index gives an idea of 

By the definition of refraction  $\mathrm{\mu }=\frac{\text{c}}{\text{v}}$ here is c is light velocity  $\left(3\times {10}^8\text{m/s}\right)$ constant so  $\mathrm{\mu }\propto \frac{\text{1}}{\text{v}}$ So it gives the idea of speed of light  or By the definition of refractive index $\mathrm{\mu }=\frac{\text{c}}{\text{v}}$  refractive index vary with speed $\left(\mathrm{\mu }\propto \frac{\text{1}}{\text{v}}\right)$