Solve Beam Divergence Problem for 1.5mW Laser

[timman_24]

[SOLVED] Beam Divergence

Homework Statement

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?

Homework Equations

Divergence=2*arctan[(Di-Df)/(2L)]

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!

 

[timman_24]

I figured it out. That equation can not be used for my problem. Here's what I did for future reference:

I found the solid angle first:
w=Af/R^2
w=(0.0025^2*pi)/(15^2)= 8.73E-8sr

After that I used it in this equation:
w=(pi*theta^2)/4

Solve for theta and divide by two gives the divergence angle.