  Anderson localization was discovered 50 years ago to describe the propagation
of electrons in the presence of disorder. The main prediction back then, was
the existence of disorder induced localized states, which do not conduct
electricity. Many years later it turns out, that the concept of Anderson
localization is much more general and applies to almost any type of propagation
in time or space, when more than one parameter is relevant (like phase and
amplitude). Here we propose a new optical scheme to literally see Anderson
localization by varying the optical wavelength or angle of incidence to tune
between localized and delocalized states. The occurrence of Anderson
localization in the propagation of light, in particular, has become the focus
of tremendous interest due to the emergence of new optical technologies and
media, such as low dimensional and disordered optical lattices. While several
experiments have reported the measurement of Anderson localization of light,
many of the observations remain controversial because the effects of absorption
and localization have a similar signature, i.e., exponential decrease of the
transmission with the system size. In this work, we discuss a system, where we
can clearly differentiate between absorption and localization effects because
this system is equivalent to a perfect filter, only in the absence of any
absorption. Indeed, only one wavelength is perfectly transmitted and all others
are fully localized. These results were obtained by developing a new
theoretical framework for the average optical transmission through disordered
media.
