What do you mean by optically denser and optically rarer mediums?

A medium ‘A’ is said to be optically denser then medium ‘B’, if the speed of light in ‘A’ is lesser than the speed of light in ‘B’ i.e the refractive index of ‘A’ is greater than the refractive index of ‘B’.

A medium ‘A’ is said to be optically rarer then the medium ‘B’, if the speed of light in ‘B’ is greater than the speed of light in ‘A’ i.e the refractive index of ‘A’ is lesser than the refractive index of ‘B’.