The ratio r of the electric displacement (electric flux per unit area) at any point in a dielectric (that is, non-conducting) medium to the displacement that an identical electric field would produce in a vacuum, measured at the same point. The relative dielectric constant, which is not in fact a constant and is more properly called the relative permittivity, is a complex number. Its imaginary part, r , is sensitive to attenuation of microwaves by absorption and other phenomena; it is sometimes called the dielectric loss. Ice, however, is generally assumed to be a low-loss medium, and its dielectric loss is approximated as r= 0. The real part of r, denoted r, depends on frequency and temperature, and more subtly on variations in crystalline fabric and the presence of impurities. It determines the geometry of wave propagation, including refraction at and reflection from interfaces between layers within the medium. See ground-penetrating radar.