The linear permittivity of a homogeneous material is usually given relative to that of free space, as a relative permittivity εr (also called dielectric constant, although this sometimes only refers to the static, zero-frequency relative permittivity). In an anisotropic material, the relative permittivity may be a tensor, causing birefringence. The actual permittivity is then calculated by multiplying the relative permittivity by ε0:
- χ (frequently written χe) is the electric susceptibility of the material.
where is the electric permittivity of free space.
The susceptibility of a medium is related to its relative permittivity by
So in the case of a vacuum,
The electric displacement D is related to the polarization density P by