The refractive index is defined as the ratio of the velocity of light in a vacuum to its velocity in a specified medium.The refractive index of diamond is 2.42 it means that it is an optically denser medium and therefore the speed of light will be less in this medium. i.e., 1.23 × 108 ms−1 (= 3 × 108 ms−1/2.42) as compared to the speed of light in vacuum.