  Shear localization in granular materials is studied experimentally and
numerically. The system consists of two material layers with different
effective frictions. The presence of the material interface leads to a special
type of "total internal reflection" of the shear zone. In a wide range of
configurations the reflection is characterized by a fixed angle which is
analogous to the critical angle of refraction in optics. The zone leaves and
reenters the high friction region at this critical angle and in between it
stays near the interface in the low friction region. The formalism describing
the geometry of the shear zones and that of refracted and reflected light beams
is very similar. For the internal visualization of shear localization two
independent experimental techniques were used (i) excavation and (ii) Magnetic
Resonance Imaging.
