Question

Measuring the distance to the moon (lunar ranging) is routinely done by firing short-pulse lasers and measuring the time it takes for the pulses to reflect back from the moon. A pulse is fired from Earth. To send the pulse out, the pulse is expanded so that it fills the aperture of a 5.40-in-diameter telescope. Assuming the only thing spreading the beam out is diffraction and that the light wavelength is 640 nm, how large will the beam be when it reaches the moon, 3.82 105 km away?

km

answer is not 3.56 km

Answer 1