Physics: Principles with Applications (7th Edition)

Published by Pearson
ISBN 10: 0-32162-592-7
ISBN 13: 978-0-32162-592-2

Chapter 28 - Quantum Mechanics of Atoms - Questions - Page 825: 26

Answer

The laser beam does not spread out appreciably.

Work Step by Step

As explained on page 820, a laser beam is very narrow and intense, and the light is a nearly perfect plane wave. Its intensity stays almost constant as it travels away from the laser. In contrast, the light from the street lamp is a spherically symmetric wave. It has an intensity that drops off as $1/r^2$ as it travels away from the streetlamp. Far away, the power from the lamp that reaches the camera is far less than 1000 W. To cite an example, at a distance of 1 km, the streetlamp’s intensity would be less than $1000W/4\pi (1000m)^2=8\times10^{-5}W/m^2$, which would be less than half the intensity of the laser at that distance. Thus the laser would appear to be stronger when photographed.
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