Calculating Focal Length and Minimum Diameter for Laser Beam Diffraction

[strikingleafs01]

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?

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 separate 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.

 

[strikingleafs01]

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

 

[meghamala]

Oh! So did you get the right answer using that formula?

 

[strikingleafs01]

nope, still nothing, anyone know anything??