[SOLVED] Beam Divergence 1. The problem statement, all variables and given/known data A 1.5mW helium-neon laser beam delivers a spot of light 5mm in diameter across a room 15m wide. The beam radiates from a small circular area of diameter 0.5mm at the output mirror of the laser. Assume that the beam irradiance is constant across the diverging beam. What is the beam divergence angle of this laser? 2. Relevant equations Divergence=2*arctan[(Di-Df)/(2L)] 3. The attempt at a solution I used the equation above and got 0.017 degrees. However the book's answer is 0.0096 degrees. It's very straight forward but for some reason its not right. I don't know if it is a typo in the book or if I am attempting to do the problem incorrectly. Any help would be greatly appreciated. Thanks!