Optical Density

General

    Like any wave, the speed of a light wave is dependent upon the properties of the medium. In the case of an electromagnetic wave, the speed of the wave depends upon the optical density of that material. The optical density of a medium is not the same as its physical density. The physical density of a material refers to the mass/volume ratio. 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.One indicator of the optical density of a material is the index of refraction value of the material.

Advanced Description

The refractive index depends on the number density of molecules and the polarizability of the molecules that make up the material. Consequently, it also depends importantly on how far the wavelength is from absorption bands in the material.

Many heavy elements have their inner electron shells shielded from electromagnetic waves by the outer ones. Really heavy elements do not contribute any more to index than lighter ones with the same number of electrons in the outer shells. So pure mass density is not always the dominant factor in the refractive index.

The chemistry and the absorption bands of the materials are important to making a good prediction about refractive index. For example, semiconductors like silicon, germanium, and diamond tend to have very high indices.

But everything else held constant, the partial derivative of of the refractive index with respect to density is positive.