Purpose:Show umbra and penumbra with an extended source, as in an eclipse.
Description:Hold the foam ball between the extended source (an incandescent-based television lightbox) and a projection screen (In the lecture halls, use the whiteboard behind the blackboards.). The shadow will consist of three distinct regions: (1) The umbra, in which all of the source is shadowed, is the dark central part. (2) The penumbra, where part of the source is shadowed, is the intermediate band around the umbra. (3) The outer region, where none of the source is shadowed, is the brightest region.