The angular width of a ruby laser beam is determined by Rayleigh’s criterion (see Problem 9-39). For this laser the diameter of the ruby rod is 1.0 cm and λ = 694.3 nm.
(a) What is the diameter of the spot projected by the ruby laser at a distance of 1.0 km?
(b) If the laser is emitting 10¹⁸ photons/s, what is the power deposited per square centimeter on the target at 1.0 km?

Data from in Problem 9-39

A laser beam is aimed at the Moon from a distance 3.84 x 10⁸ m away. The angular spread of the beam is given by the diffraction formula (Rayleigh’s criterion), sin θ = 1.22λ/D, where D is the diameter of the laser tube or rod.