²⁵The “index of refraction” (n) for any substance is the ratio of the speed of light through a vacuum (c) compared to the speed of light through that substance (v): n = c v. For all substances this value will be greater than one (i.e. the speed of light will always be greatest through a vacuum, at 299792458 meters per second or 186282.4 miles per second). Thus, the refractive index for an optically transparent substance is analogous to the reciprocal of the velocity factor of an electrical transmission line, where the permittivity and permeability of the cable materials act to slow down the propagation of electric and magnetic fields through the cable.