  We admit that the vacuum is not empty but is filled with continuously
appearing and disappearing virtual fermion pairs. We show that if we simply
model the propagation of the photon in vacuum as a series of transient captures
within the virtual pairs, we can derive the finite light velocity $c$ as the
average delay on the photon propagation. We then show that the vacuum
permittivity $\epsilon_0$ and permeability $\mu_0$ originate from the
polarization and the magnetization of the virtual fermions pairs. Since the
transit time of a photon is a statistical process within this model, we expect
it to be fluctuating. We discuss experimental tests of this prediction. We also
study vacuum saturation effects under high photon density conditions.
