 Specifically, in Weyl gauge formula_37 because the period of formula_29 for ultraviolet light is about two orders of magnitude larger than the period of the electron's wave function.In both gauges it is assumed that the electrons at the surface had little time to respond to the incoming perturbation, and add nothing to either of the two potentials.Moreover, it is for most practical uses safe to neglect the quadratic formula_39 term.