Refractive index of medium A with respect to medium B = speed of light in medium B/speed of light in medium A
nAB = vB/vA
When the medium B is vacuum, vB = c the speed of light in vacuum.
nA =c/vA
This constant is called the absolute refractive index of medium A.
Refractive index of medium A with repect to medium B can also be calculated as:
nAB= nA/nB
Where nA and nB are absolute refractive index of medium A amd B respectively.