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?
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.
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.