where η_i is the refractive index at a specified frequency i, Vi is the velocity of the radiation in the medium, and c is its velocity in a vacuum. The refractive index of most liquids lies between 1.3 and 1.8; it is 1.3 to 2.5 or higher for solids.4
The interaction involved in transmission can be ascribed to periodic polarization of the atomic and molecular species that make up the medium. Polarization in this context