What is the Unit of the Refractive Index?

Answer:

The ratio of the speed of light in a vacuum to the light speed in a medium. Refractive index has no specific unit. Because the refractive index of a medium represents the ratio of the speed of light in a vacuum of light speed of the same light within the medium, and ratios do not have any unit. 

The refractive index is used to understand that light enters another material, and how much of the path is bent or refracted through which the light is traveling.