Laser beam diffraction

strikingleafs01

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.

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??