Light signaling has long been used to communicate between

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Question: Light signaling has long been used to communicate between ships on the open sea privately. The USS Nimitz is trying to communicate secretly to one of its protective destroyers using a light signal. If this is done with focusing OPTICS producing a 0.6-m diameter beam of 600-nm wavelength light,

Question 1: What is the minimum angular spread of the beam?

I want you to explain the physics in this problem.