Light trap - Lycée Żmichowska (Varsovie) Lycée Vicat (Souillac)

Consider a plane figure f made of two sides AB and BC of a triangle and a ray of light sliding into f. Determine a concrete example of points A, B, C and a ray of light , so that we obtain 4, 5, 6, … n reflections. Is it possible, that the ray never comes out, that is to say, the number of reflections is infinite? ─ That would mean that our simple figure f is really a light trap... If you manage to solve these problems, we may think similar problems for slightly more complex traps ...