Virtual images and billiards
  Students in introductory physics courses struggle to understand virtual image formation by a plane mirror and the proper construction of ray diagrams. This difficulty, if not sufficiently addressed, results in further problems throughout the study of geometric optics. Specifically, students fail to apply proper graphical representation of light rays during investigations of the formation of real images by converging lenses and concave mirrors. We present a guided-inquiry activity based on the research-verified Physics by Inquiry text that incorporates a small and inexpensive billiards table, with billiard balls acting as "light". In this way, we approach the abstract concept of virtual images by relation to the concrete concept of physical reflection. 
J. Christopher Moore, Richard D. Floyd and Cody V. Thompson
physics.ed-ph