Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 26 - Think and Explain: 66

Answer

The concept is explained in Figure 26.13.

Work Step by Step

The sun has a finite extent and is not a point source of light. When the plane is flying at a low altitude, its shadow is sharp because light coming from different parts of the sun doesn't get a chance to spread out much behind it before it hits the Earth. The shadow of a high-flying plane is filled in mainly by light tapering in from different parts of the Sun. This is the same effect responsible for the penumbra of a solar eclipse (page 495, Figure 26.15).
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