1. The problem statement, all variables and given/known data A scientist needs to focus a helium-neon laser beam (lamda = 633nm) to a 10 micrometre diameter spot 8.0 cm behind the lens. a) What focal-length should she use? b) What minimum diameter should she use? 2. Relevant equations w = (2.44lambda * focal length) / diameter theta = 1.22lamda/diameter Wmin is apprx equal to 2.5lambda....apparently, but if you have a seperate method ignore this. 3. The attempt at a solution i attempted to calculate w through 2.5lambda, then use that to calculate the focal length. the answer i got was 0.001cm.
I cannot seem to find any modification of this equation anywhere, but it seems that this is the only equation that is used to solve this problem