The momentum of light in a refracting medium

Abstract It is shown that neither Minkowski’s result, according to which the ratio of momentum to energy for a light wave in a medium of refractive index n is n/c, nor that of Abraham, who found 1/nc, is correct. For a broad wave in a uniform medium, the correct answer is given by (2.12) with σ = 1/5. For weak refraction it is approximately equal to the average of the Abraham and Minkowski results. Abraham’s formula gives correctly the part of the momentum which resides in the electromagnetic field, but not the mechanical momentum of the medium which travels with the light pulse. Minkowski’s formula gives the pseudo-momentum, a quantity of physical interest. The momentum change upon reflexion or transmission usually involves also acoustic transients, these are discussed for some simple cases.