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Homework Help: Light Questions - Please Help

  1. Feb 2, 2005 #1
    Light Questions - Please Help!!!

    I thought I was doing this problem right, but according to webassign I'm not, and I can't figure out what I've done wrong... please help!!

    1. A helium-neon laser emits light that has a wavelength of 632.8 nm. The circular aperture through which the beam emerges has a diameter of 0.500 cm. Estimate the diameter of the beam 16.0 km from the laser.

    >> Here's what I did:
    min angle = d/L = .005/16000 = 3.125e-7
    min angle = 1.22(lamda/D)
    -> min angle/1.22 = lambda/D
    3.125e-7/1.22 = 6.328e-7/D
    -> 6.328e-7/2.56e-7 = D = 2.47

    Not sure why this is wrong... :cry:

    2. The second-order bright fringe in a single-slit diffraction pattern is 1.50 mm from the center of the central maximum. The screen is 80.0 cm from a slit of width 0.600 mm. Assuming that the incident light is monochromatic, calculate the light's approximate wavelength.

    >> I'm not really sure how to start this at all. I know I'm looking for lambda and that a=6.0e-4m, L=0.800m, m=2

    Any ideas??
  2. jcsd
  3. Feb 2, 2005 #2
    For the second question, use this formula:

    [tex]\lambda = (\frac{x}{L}) (\frac{d}{n-\frac{1}{2}})[/tex]
  4. Feb 2, 2005 #3
    I plugged in that formula and got a wavelength of 750 nm... which webassign is still saying is wrong...

    Ack Im so confused!! :eek:
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