  In this paper we show that a free electromagnetic field living in Minkowski
spacetime generates an effective Weitzenbock or an effective Lorentzian
spacetime whose properties aredetermined in details. These results are possible
because we found using the Clifford bundle formalism the noticeable result that
the energy-momentum densities of a free electromagnetic field are sources of
the Hodge duals of exact 2-form fields which satisfy Maxwell like equations.
