To account for this situation and other nonlocal effects our description must be improved. The next step in considering the spatial variations of the fields over the polarizable unit is then to consider the magnetic dipoles and electric quadrupoles that are induced by the applied fields. They will oscillate and emit radiation which adds to the contribution from the oscillating electric dipoles. However, instead of introducing new source terms in Maxwell's equations, that should be added to the wave equation in Eq. This generalized polarization, which also will be denoted by P , is no longer a local function of the electric field.
H is the magnetic field and can be both an externally applied dc field or an optical magnetic field.
Though, if H is an optical field it is related to the corresponding electric field through Maxwell's equations and can be incorporated into the term . Hence, we can assume that H is an external static magnetic field in Eq. A temporal nonlocality is still implicit in Eq. If we, in the spirit of Eq. The new linear term in the polarization, proportional to , gives rise to an effect called natural optical activity or natural gyrotropy.