Why the speed of light is reduced in a transparent medium

It is well known from optics that the speed of light in a transparent medium is reduced by a factor of n (the index of refraction) as compared with vacuum. Maxwell’s electrodynamics provides a simple account of this phenomenon, and relates n to the electric susceptibility of the material. But the conventional analysis does little to illuminate the mechanism involved. This paper offers some elucidation of the ‘‘miracle’’ by which the radiation from many induced molecular dipoles conspires to produce a single wave propagating at the reduced speed.