  We argue that in fat brane-world scenarios the light propagating in vacuum
will, because of massive ``Kaluza-Klein'' (KK) excitations, experience a
refraction. The motion of a photon inside a fat brane can be decomposed in the
longitudinal and transverse directions with respect to the surface of the
brane. Since the light observable propagation is related only with the
longitudinal motion, the observed speed of light depends on the value of the
momentum transverse fraction contributing as the massive KK excitations. This
is directly connected with the energy of the particles emitting the light, and
hence with the frequency of the light itself. Using recent results on the
arrival times of radiation of different energies from the measurements of
gamma-ray bursters with known redshifts, we establish the limit M>620 TeV on
the inverse thickness of the brane, and thus on the masses of the KK
excitations. This limit exceeds by at least one order of magnitude the typical
energy scale currently in use to characterize brane phenomena in the realm of
future colliders.
