At the high-frequency limit (meaning optical frequencies), the complex permittivity is commonly referred to as ε ∞ (or sometimes ε opt). At the plasma frequency and below, dielectrics behave as ideal metals, with electron gas behavior. The static permittivity is a good approximation for alternating fields of low frequencies, and as the frequency increases a measurable phase difference δ emerges between D and E. The frequency at which the phase shift becomes noticeable depends on temperature and the details of the medium. For moderate field strength (E o), D and E remain proportional, and