PURPOSE: To show umbra and penumbra with an extended source, as in an eclipse.

DESCRIPTION: Hold the foam ball between the extended source 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.

SUGGESTIONS: Change the nature of the shadow by moving the foam ball back and forth between the source and the screen.

REFERENCES: (PIRA 8A10.30)

EQUIPMENT: Circular extended source in TV light box with foam ball on dowel.

SETUP TIME: None.