Light-Reflection and Refraction
Refractive index is the ratio of the velocity of light in a vacuum to its velocity in a specified medium.
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how light propagates through that medium.
Since, every material is composed of different structure, hence when light passes through it its velocity will change as per the composition of material.
Ex: refractive index of air is 1, of water is 1.33, of diamond 2.42, etc.