Polarizability for anisotropic or non-spherical media cannot in general be represented as a scalar quantity. Defining formula_2 as a scalar implies both that applied electric fields can only induce polarization components parallel to the field and that the formula_19 and formula_20 directions respond in the same way to the applied electric field. For example, an electric field in the formula_21-direction can only produce an formula_21 component in formula_4 and if that same electric field were applied in the formula_24-direction the induced polarization would be the same in magnitude but appear in the formula_24 component of formula_4. Many crystalline materials have directions that are easier to polarize than others and some even become polarized in directions perpendicular to the applied electric field, and the same thing happens with non-spherical bodies. Some molecules and materials with this sort of anisotropy are optically active, or exhibit linear birefringence of light.