  Exact analytical solution for the space-time evolution of electromagnetic
field in electrically conducting nuclear matter produced in heavy-ion
collisions is discussed. It is argued that the parameter that controls the
strength of the matter effect on the field evolution is $\sigma\gamma b$, where
$\sigma$ is electrical conductivity, $\gamma$ is the Lorentz boost-factor and
$b$ is the characteristic transverse size of the matter. When this parameter is
of the order one or larger, which is the case at RHIC and LHC, space-time
dependence of electromagnetic field is completely different form that in
vacuum.
