To understand the relationship between refractive index and the atomic or molecular structure of a material it is
necessary to recall that light can be treated as a varying electric field. If a static electric field is applied to an
insulating material, the internal components which carry a charge will try to line up with the field and the
material is said to become polarised. This means that any positively and negatively charged species present are
rearranged slightly, with the positive charges moving in the field direction and the negative changes against it.
Hence, some parts of the material take on a slightly positive charge while an equal number of parts become
negative. The extent of this separation is measured as the relative permittivity (formerly dielectric constant) of
the substance. The magnitude of the relative permittivity is found to be closely related to that of the refractive
index.