Electromagnetic momentum in a material medium

When an electromagnetic wave, in free space, is incident upon a totally absorbing surface, then, as shown by Maxwell, there is a radiation pressure on the surface given by P = W, where W is the field-energy density in the wave. This is consistent with the concépt of a flux of energy in the wave given by Poynting´s vector S = E×H and having a momentum per unit volume p = mc = D×B, where m = W/c2 is the mass of the energy W. For a wave in a material medium of constants ¿, ¿ (relative permittivity and relative permeability), however, although the radiation pressure P = W is consistent with a Poynting energy flux E×H and an equivalent momentum D×B, the latter is greater than the momentum of the moving field energy. It is shown that the explanation of this discrepancy lies in a stress acting on the material medium itself, and that the true electromagnetic momentum is given by E×H/c2