  The radiation emitted by a black hole (BH) during its evaporation has to have
some degree of quantum coherence to accommodate a unitary time evolution. We
parametrize the degree of coherence by the number of coherently emitted
particles $N_{coh}$ and show that it is severely constrained by the equivalence
principle. We discuss, in this context, the fate of a shell of matter that
falls into a Schwarzschild BH. Two points of view are considered, that of a
stationary external observer and that of the shell itself. From the perspective
of the shell, the near-horizon region has an energy density proportional to
$N_{coh}^2$ in Schwarzschild units. So, if $N_{coh}$ is parameterically larger
than the square root of the BH entropy $S_{BH}^ {1/2}$, a firewall or more
generally a "wall of smoke" forms and the equivalence principle is violated
while the BH is still semiclassical. To have a degree of coherence that is
parametrically smaller than $S_{BH}^{1/2}$, one has to introduce a new
sub-Planckian gravitational length scale, which likely also violates the
equivalence principle. And so our previously proposed model which has
$N_{coh}=S_{BH}^{1/2}$ is singled out. From the external-observer perspective,
we find that the time it takes for the information about the state of the shell
to get re-emitted from the BH is inversely proportional to $N_{coh}$. When the
rate of information release becomes order unity, the semiclassical
approximation starts to break down and the BH becomes a perfect reflecting
information mirror.
