Press space bar to start and button "Lines" for help (click and drag the bleu dot on circle...). The problem is called the billiard problem because it corresponds to finding the point on the edge of a circular "billiard" table at which a cue ball at a given point must be aimed in order to carom once off the edge of the table and strike another ball at a second given point. The problem was first formulated by Ptolemy in 150 AD, and was named after the Arab scholar Alhazen, who discussed it in his work on optics...
