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Homework Help: Laser Diffraction

  1. Feb 26, 2006 #1


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    A helium neon laser ([tex] \lambda [/tex] = 633nm. is built out of a glass tube 1.0mm, 1 X 10^-3m, in diameter. One mirror is partialy transparent letting the laser beam out. An electrical discharge causes the tube to glow like a neon light. From ans optical perspective, the laser beam is a light wave that diffracts out through a 1.0mm diameter circular opening.

    a) can the beam ever be perfectly parallel? No because there will always be diffraction.
    b) Whst is the minimum divergence angle, [tex] \theta_1[/tex], of the beam.

    This is a circular aperture so the angle of divergence is:

    [tex] \frac{1.22\lambda}{D} [/tex] where D is the diameter of the aperture.

    So the angle is [tex] \theta_1 = \frac{1.22(633X10^{-9})}{1X10^{-3}} = 7.7X10^{-4} radians.[/tex]

    The above answer is apparently wrong but I can't figure out why.

    c) What is the diameter of the laser beam after it has traveled 3.0m

    [tex] w = \frac{2.44\lambda L}{D} = .004m[/tex]
    This is also wrong. The actual answer is .002m, but again. I don't know why.

    d) the diamter after 1.0km? This involes the same method as part c and agin I got it wrong. What am I missing here thats screwing me up. Thank you for the help.
  2. jcsd
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