  The amplitude of localized quantum states in random or disordered media may
exhibit long range exponential decay. We present here a theory that unveils the
existence of an effective potential which finely governs the confinement of
these states. In this picture, the boundaries of the localization subregions
for low energy eigenfunctions correspond to the barriers of this effective
potential, and the long range exponential decay characteristic of Anderson
localization is explained as the consequence of multiple tunneling in the dense
network of barriers created by this effective potential. Finally, we show that
the Weyl's formula based on this potential turns out to be a remarkable
approximation of the density of states for a large variety of one-dimensional
systems, periodic or random.
