Electrons have very much smaller mass than that of ions, so they respond more rapidly to a changing electric field. For electric field that oscillates at extremely high frequencies (like light) only electronic polarization can takes place. At lesser frequencies, the relative displacement of positive and negative ions can takes place. Orientation of permanent dipoles, which require the rotation of a molecule can occur only if the oscillation is relatively show (MHz range or slower). The total polarization P, total polarizability and the relative permittivity of a dielectric in an alternating field all depend on the ease with which the dipoles can reverse alignment with each other reversal of the field. Some polarizability mechanisms do not permit sufficiently rapid reversal of the dipole alignment. In such a process the time required to reach the equilibrium orientation is called the relaxation time and its reciprocal the relaxation frequency. It can be also defined as the time specific polarization to occur is called the relaxation time. When the frequency of the applied field exceeds that of the relaxation frequency of a particular polarization process, the dipoles cannot reorient fast enough and operation of the process ceases.