polarization [@Baier_1976_b] and photon splitting [@Di_Piazza_2007] in a laser field are two examples of refractive QED processes, which have been considered in the literature. It has been observed in both cases, that the net number of laser photons exchanged with the laser field is very small (of the order of unity) even in the ultra-relativistic limit $\xi\to \infty$. As a general remark to be kept in mind throughout in the paper, we observe that the laser field is treated as a classical field in those papers and here as well. Thus, an expression like “the net number of laser photons exchanged with the laser field is very small” has to be intended more precisely as “the net energy and momentum exchanged with the laser field is very small, corresponding to a few laser photons.”

In the present paper, by analyzing the amplitude of a general refractive QED effect, we indicate analytically that this is a general feature of such processes in a strong laser field. The physical origin of this effect lies in the fact that in a refractive QED process, the laser field couples to the external photons only indirectly via a virtual electron-positron pair. As we will see 