Tiling billiards is a dynamical system where beams of light refract through planar tilings. It turns out that, for a regular tiling of the plane by congruent triangles, the light trajectories can be described by interval exchange transformations. I will explain this surprising correspondence, give related results, and show computer simulations of the system.

Periodic trajectories in polygons are rare and beautiful, and have several surprising properties. During the lockdown period in 2020, we wrote a Sage program to draw every periodic trajectory on every regular polygon with an odd number of edges. I'll explain the underlying structure, and show lots of pictures from this vast menagerie. This is joint work with Samuel Lelièvre.