If = 533 , what is the minimum diameter of the circular opening from which the l

Question

If = 533 , what is the minimum diameter of the circular opening from which the laser beam emerges? The earth-moon distance is 3.84×105 .

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 it 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 in diameter. Staying within this diameter is accomplished by using a special large-diameter laser.

Explanation / Answer

wavelength w = 533 nm= 533 * 10 ^-9 m The earth-moon distance is D = 3.84×10^5 km D = 3.84 * 10 ^8 m resolution x = 1 in = 2.54 cm = 2.54*10^-2 m we know x = 1.22 wD / d from this diameter of the opening from which the laser beam emerges d = 1.22wD/x d = 9830.7 m

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