Question

The refractive index of diamond is 2.42. What is the meaning of this statement?

Answer 1

Refractive index of a medium nm is related to the speed of light in that medium v by the relation: 

\(n_m = \frac{\text{Speed of light in air }}{\text{Speed of light in the medium }}= \frac{c}{ v}\)

Where, c is the speed of light in vacuum/air. 
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.