how can we say that the refractive index of a medium is inversely proportional to the velocity of light in that medium?

The refractive index of a medium is a measure of its optical density. The optical density of a material relates to the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.