Helium-laser beam question

  • #1
1. Homework Statement

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. Homework 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.
 

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