The ability of a medium to refract light is also expressed in terms of optical density. Optical density has a definite connotation. It is not the same as mass density. We have been using the terms 'rarer medium' and 'denser medium'. It actually means 'optically rarer medium' and 'optically denser medium', respectively. The one with the larger refractive index is optically denser medium than the other. The speed of the light is higher in a rarer medium than in a denser medium.