An effect observed in a dielectric in the presence of an applied electric field. The electrons in each particular atom are displaced in the direction opposite to the field, and the nucleus in the direction of the field. (The centre of gravity of the atom remains fixed.) Each atom acquires a dipole moment (see dipole) parallel to and in the same direction as the field. The polarization is defined as the electric dipole moment per unit volume. It is related to the electric field, E, by the relation

where χ_e is the electric susceptibility and ε₀ the permittivity of free space. χ_e is a tensor and approximately independent of electric field strength in most practical circumstances. In a uniform isotropic medium P and E are parallel and χ_e is a scalar constant.