The refractive index of diamond is

solution:
We know that, absolute refractive index (n) of a medium is given by,

$n=\frac{c}{v}$ or $v=\frac{c}{n}$ i.e., $v \propto \frac{1}{n}$

Since, the refractive index of diamond is 2.42, this suggests that the speed of light in diamond will reduce by a factor 2.42 compared to its speed in air.