  We study strain localization in slow shear flow focusing on layered granular
materials. A heretofore unknown effect is presented here. We show that shear
zones are refracted at material interfaces in analogy with refraction of light
beams in optics. This phenomenon can be obtained as a consequence of a recent
variational model of shear zones. The predictions of the model are tested and
confirmed by 3D discrete element simulations. We found that shear zones follow
Snell's law of light refraction.
