Conceptual Physics (12th Edition)

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

Chapter 31 - Think and Explain: 39

Answer

See below.

Work Step by Step

Some automatic doors use a beam of light that continuously shines on a light-detecting surface. Some of these "photodetectors" rely on the photoelectric effect - as long as the light shines, electrons are ejected and the resulting small current is amplified. When someone walks through the beam of light, the small current stops. The door-opening circuit is triggered and opens the door.
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