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Homework Help: Laser beam diffraction

  1. Dec 1, 2007 #1
    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.
  2. jcsd
  3. Dec 1, 2007 #2
    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
  4. Dec 1, 2007 #3
    Oh! So did you get the right answer using that formula?
  5. Dec 1, 2007 #4
    nope, still nothing, anyone know anything??
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