  TeV photons from blazars at relatively large distances, interacting with the
optical-IR cosmic background, are efficiently converted into electron-positron
pairs. The produced pairs are extremely relativistic (Lorentz factors of the
order of 1e6 1e7 and promptly loose their energy through inverse Compton
scatterings with the photons of the microwave cosmic background, producing
emission in the GeV band. The spectrum and the flux level of this reprocessed
emission is critically dependent on the intensity of the intergalactic magnetic
field, B, that can deflect the pairs diluting the intrinsic emission over a
large solid angle. We derive a simple relation for the reprocessed spectrum
expected from a steady source. We apply this treatment to the blazar 1ES
0229+200, whose intrinsic very hard TeV spectrum is expected to be
approximately steady. Comparing the predicted reprocessed emission with the
upper limits measured by the Fermi/Large Area Telescope, we constrain the value
of the intergalactic magnetic field to be larger than $B \simeq 5\times
10^{-15}$ Gauss, depending on the model of extragalactic background light.
