Using the knowledge of penumbra with a trick simulation
  The study is about a basic shadow experiment, which was enriched with a simulation to understand the reasoning of participants when we use a trick. Two light sources create an umbra and penumbra behind the objects. With this experiment, we asked what would happen when the penumbras interact. Most of the participants predicted the correct solution, that there should be an umbra. Some of the participants choose wrong alternative, and explained in terms of the structure of penumbra. 
Mustafa \c{S}ahin B\"ulb\"ul and Loo Kang Wee
physics.ed-ph