As well as the other excellent answers, try ray tracing, 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]

As well as the other excellent answers, try ray tracing, 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), [https://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]