Question

What is Refractive index?

Answer 1

In optics, the refractive index or index of refraction n of a material is a dimensionless number that describes how light propagates through that medium. It is defined as
N=c\v
where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than it does in water.

Answer 2

absolute refractive index is defined as a ratio of the speed of light in the vacuum and in selected medium  

relative refractive index is defined as a ratio of speed of light in two different media

When in relative refrective index ,second medium is vacuum then both will be same