Editing Permittivity (section)

=== Quantum-mechanical interpretation ===
In terms of [[quantum mechanics]], permittivity is explained by [[atom]]ic and [[molecule|molecular]] interactions.

At low frequencies, molecules in polar dielectrics are polarized by an applied electric field, which induces periodic rotations. For example, at the [[microwave]] frequency, the microwave field causes the periodic rotation of water molecules, sufficient to break [[hydrogen bond]]s. The field does work against the bonds and the energy is absorbed by the material as [[heat]]. This is why microwave ovens work very well for materials containing water. There are two maxima of the imaginary component (the absorptive index) of water, one at the microwave frequency, and the other at far [[ultraviolet]] (UV) frequency. Both of these resonances are at higher frequencies than the operating frequency of microwave ovens.

At moderate frequencies, the energy is too high to cause rotation, yet too low to affect electrons directly, and is absorbed in the form of resonant molecular vibrations. In water, this is where the absorptive index starts to drop sharply, and the minimum of the imaginary permittivity is at the frequency of blue light (optical regime).

At high frequencies (such as UV and above), molecules cannot relax, and the energy is purely absorbed by atoms, exciting [[electron]] energy levels. Thus, these frequencies are classified as [[ionizing radiation]].

While carrying out a complete ''[[ab initio]]'' (that is, first-principles) modelling is now computationally possible, it has not been widely applied yet. Thus, a phenomenological model is accepted as being an adequate method of capturing experimental behaviors. The [[Debye relaxation|Debye model]] and the [[Lorentz model]] use a first-order and second-order (respectively) lumped system parameter linear representation (such as an RC and an LRC resonant circuit).