The formulation of the momentum of electromagnetic radiation or photons in media is a historical question. After Peierls discussed all these options, a crucial new value for the dominating dispersion factor σ was derived for non-ionized media with the consequence that an opto-acoustic coupling exists. After the experimental confirmation of the Kapitza-Dirac effect the generalization of the effect for quantum modulation of electron beams by crossing laser beams is ascertained. The evaluation of repeated experiments of this quantum modulation is very precisely confirming by measurements that the Peierls factor σ = 1/5 is correct.
