1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Laser Diffraction

  1. Feb 26, 2006 #1


    User Avatar
    Homework Helper
    Gold Member

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted