I am somewhat confused about the connection between divergence and solid angle for a beam. I know individually what each term means... but I'm confused as to how (or even if) one can calculate the solid angle of a beam, given the divergence. I have some notes from a previous lecture series I attended on laser physics, where I wrote down: "Laser radiation has a narrow divergence (~1mrad), and so a small solid angle (~10^-6∏ sterad)". That seems to imply that the divergence is sufficient information to calculate the solid angle of the beam... but I just can't seem to do it. (I'm not concerned just with the numbers given here, but rather the general problem of how to relate solid angle and beam divergence) Thanks!