As well as the other excellent answers, try [ray tracing][1], both in physics and engineering [propagation of waves (electrons, light, sound, ...)] and computer graphics [propagation of light for realistic image generation].

My survey article on the Lorentz gas (billiards in extended space), [http://arxiv.org/abs/1402.7010 and Commun. Theor. Phys.] gives a few more applications; the final paragraph reads

***Physics of Transport** Finally, the Lorentz gas and similar
models have often been used to model transport on
small scales. In this context, the use of polygonal channels
for studying nanopores was mentioned in Subsec. 6.4.
Lorentz channels have been used to understand thermoelectric
efficiency.[220] Other examples have included confined fluids,[221−222] glasses,[7] nuclear collisions,[223] and
zeolites.[224]*



  [1]: http://en.wikipedia.org/wiki/Ray_tracing