Question

What is Refractive index?

Answer 1

The extend to which bending of light take place is called refractive index.
Example:-Take a bucket full of water.
Now dip a ruler into it.
You will observe that the ruler become curve

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

Formula n=c/v