Backscattering cone

In a stationary and non-magnetic material light propagation possess time-reversal symmetry. This means that the same random path in a disordered medium will always yield the very same phase shift. Therefore those parts of the incident light that experience the same random walk will always emerge from the medium perfectly in phase and will always interfere constructivly. This simple fact has the striking consequence that, in the exact beckscattering direction, there is always the contribution to reflected intensity coming from counter-propagating paths. As a result the intensity in the exact backscattered direction is double with respect to the incohernet (diffusive) contribution and a narrow cone of enhanced reflection is produced.coherent backscattering cone diagrams
The width and the shape of this cone is strongly dependent from the microscopic properties of the scattering medium. In particular the width of the cone depends linearly on the scattering mean free path (or the transport mean free path if the scattering is not isotropic). Therefore the complete measurement of the backscattering cone shape can be used (and it is, in fact, commonly used) to characterize the scattering properties of disordered systems.