Conceptual Physics (12th Edition)

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

Chapter 29 - Think and Explain: 46

Answer

If the film is too thick, the beams reflected from the front and back surfaces will not overlap.

Work Step by Step

See the diagram. For interference to occur, two waves must coincide. If the film is too thick, the wave that reflects from the top surface is displaced from the wave that reflects from the bottom surface. Because they do not overlap, there is no interaction, and no interference or iridescence occurs.
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