Question

A He-Ne laser having an emergent beam diameter of 1 cm is flashed on to the moon surface. If the distance between the earth and moon is 3,84,403 km, what would be the diameter of the laser beam spot on the moon?

Answer 1

$\theta=1.22\lambda/d=1.22\cdot632.8\cdot10^{-9}/0.01=0.0000772\ (rad)$

$D=2\cdot l\cdot\tan\theta=2\cdot384403000\cdot\tan0.0000772=59353\ (m)$ . Answer