Chapter 33, Problem 65EAP

Scientists use laser range-finding to measure the distance to the moon with great accuracy. A brief laser pulse is fired at the moon, then the time interval is measured until the “echo” is seen by a telescope. A laser beam spreads out as it travels because it diffracts through a circular exit as il leaves the laser. In order for

the reflected light to be bright enough to detect, the laser spot on the moon must be no more than 1.0 km in diameter. Staying within this diameter is accomplished by using a special large-diameter laser. If $\lambda \text{ = 532}$ nm, what is the minimum diameter of the circular opening from which the laser beam emerges? The earth-moon distance is 384.000 km.