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Because polarization cannot follow an electric field in a high-frequency field, permittivity has a dependence on the frequency. This dependence is called dielectric dispersion. It is very important for the application of dielectric materials and the analysis of polarization systems.

When the frequency becomes higher:

1. it becomes impossible for dipolar polarization to follow the electric field in the microwave region around 10¹⁰ Hz;
2. in the infrared or far-infrared region around 10¹³ Hz, ionic polarization loses the response to the electric field;
3. electronic polarization loses its response in the ultraviolet region around 10¹⁵ Hz.

In the wavelength region below ultraviolet, permittivity approaches the constant ε₀ in every substance, where ε₀ is the permittivity of the free space. Because permittivity indicates the strength of the relation between an electric field and polarization, if a polarization process loses its response, permittivity decreases.

See also

• Dielectric relaxation
• Dielectric spectroscopy