$($a$)$ A laser with a $0\mathrm{.}03-$mrad divergence, $1000$ W of power, and $2-$cm exit diameter is aimed at the moon $(238,000$ mi$).$ What is the beam size on the moon? What is the

radiation intensity?

$($b$)$ Suppose a $700-$nm beam with a power of $0\mathrm{.}2$ mW and a $5-$cm diameter strikes a detector with a $0\mathrm{.}2-$cm diameter. How many photons strike the detector per second?