The middle, totally dark part of a shadow is called the umbra. The partial shadow, the less-dark part at the edges, is called the penumbra.
Work Step by Step
This is discussed on pages 493-495, and shown in Figures 26.12 and 26.13.
You can help us out by revising, improving and updating this answer.Update this answer
After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.