$(4$b$)$ If this Laser beam is found to be $-12$ dB from its on$-$axis value of power at a measure angle of $60.$ If the angular power dependence is P$(0)=$ Pocos"$0,$ where $0$ is angle measured from the on$-$axis position and Po is the on$-$axis power, find the full$-$angle beam divergence of this source.