Conceptual Physics (12th Edition)

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

Chapter 29 - Think and Discuss: 65

Answer

They produce a spot that is bright.

Work Step by Step

One wave travels 2 full wavelengths longer than the other wave. Because the waves were initially in phase, delaying one of them by 2 full wavelengths means they are still in phase. This results in constructive interference. This is discussed on pages 549-551, and shown in Figure 29.12a. Delaying one wave by 2 wavelengths doesn't change the fact that they are in phase.
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