The index of refraction of a material is the ratio between the speed of light in vacuum and the speed of light in that material:
[tex]n= \frac{c}{v} [/tex]
where
n is the refractive index
[tex]c=3 \cdot 10^8 m/s[/tex] is the speed of light in vacuum
v is the speed of light in the material
For sapphire, n=1.76, so if we re-arrange the previous equation and we use this value, we can calculate the speed of light in sapphire:
[tex]v= \frac{c}{n}= \frac{3 \cdot 10^8 m/s}{1.76}=1.7 \cdot 10^8 m/s [/tex]
