Teaching diffuse, specular, and total internal reflection via a halo effect

Abstract Interesting and pedagogically useful physics can be found in the halo produced by a laser pointer directed into a shallow body of water. We describe a simple model for this phenomenon using geometric optics and support this model with empirical evidence. We also discuss and verify extensions to this model that explain variations in the visual pattern. These variants include a double halo pattern and a disk of dim light that can be produced with minor modifications of the single halo configuration. Although the level of complexity differs, all of these models rely on the same ray optics, specifically refraction and diffuse, specular, and total internal refection. We discuss several scenarios for integrating halo effects into undergraduate physics instruction. This phenomenon offers instructors a realistic application of many introductory optics concepts packed into single effect that is practical and appropriate for multiple teaching environments.