1. The problem statement, all variables and given/known data A 655nm gaussian laser beam has a waist of 15mm located at a lens with focal length of 12 cm. What is the minimum beam waist and where is it located? 2. Relevant equations Beam radius w = w0*sqrt(1-(z/zr)^2) w0 = min beam waist; z = distance, zr = Rayleigh range 3. The attempt at a solution I'm attempting to find the distance from where w0 is located to the focal point, so I'd be able to calculate the distance R1 to the lens. Then perhaps calculate R2 from (1/R1)-(1/R2)=(1/f), where I assume the minimum w0 is located. Problem is I have no idea how to find this initial distance along the wave front of the beam.