Why is the speed of light in the vacuum c=299 792 458 m/s? Being a limit velocity it is intuitive to imagine that nature has in itself an intrinsic mechanism that does not allow to exceed this value. Is it then possible to think that the speed of light is greater than c and that vacuum conditions impose this limit? In this work I want to show that there is a law based on the energy density of vacuum and linked to nature constants that highlights how the viscosity of quantum vacuum "forces" photons to travel at limit speed c as a body falling into the Earth's atmosphere due to viscous friction. Furthermore, the deflection of photons that pass close to a mass such as that of the Sun, can be attributed to the refraction produced by the different energy densities of the quantum vacuum in interstellar space and in the space on the surface of a physical object?