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Diffraction grating and Dirac comb

  1. Nov 3, 2014 #1
    1. The problem statement, all variables and given/known data
    I need to measure (with the ruler) the width the depicted sinc envelope and the period of the depicted Dirac comb light pattern.

    And from the above I need to calculate the width of one slit a (i.e. aperture width) of the grating, and the period of the grating dx (i.e. distance between each slit). I have no idea where to start :(
    diffraction.jpg


    2. Relevant equations


    3. The attempt at a solution
    I suck at physics and spent few hours reading everything I could find on the internet on convolution, diffraction and fourier theorem. I still have no clue what I need to measure!
     
  2. jcsd
  3. Nov 3, 2014 #2

    vela

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    What quantities can you measure from the diffraction pattern?
     
  4. Nov 3, 2014 #3
    distance between each point? I really don't know :/
     
  5. Nov 3, 2014 #4
    I forgot to mention the distance between the slit and the points on the paper was 1000mm and the wavelength was 633nm. I tried to calculate width of one slit a = 9Lλ/0.31 = 9(1000)0.000633/0.31= 18.3mm
     
  6. Nov 3, 2014 #5

    vela

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    Which of the six figures does the photo of the diffraction pattern correspond to?
     
  7. Nov 3, 2014 #6

    vela

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    You mention two distances. Which one was 1000 mm? Do you realize that 1000 mm is 1 meter? I doubt the slits or the points on the paper were 1 meter apart.
     
  8. Nov 4, 2014 #7
    There was 1 metre distance between the slit frame and the photographic paper that recorded the diffraction. 1 metre allowed the points to be perfectly focused. Six figure distraction is corresponding to the last image on the right I think, and I'm guessing the middle one above it corresponds to the slits and the distance between them?
     
  9. Nov 6, 2014 #8

    vela

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    Ah, okay, I totally misread what you wrote. I took it to mean the distance between the points was 1 m or the distance between the slits was 1 m whereas you meant the distance from the slits to the screen is 1 m.

    You're right that the diffraction pattern corresponds to the figure on the bottom right, so by measuring the distance between the points, you can get a measure of what ##\delta \omega## is. Since ##\delta x## and ##\delta \omega## are related, you can, in principle, be able to figure out what ##\delta x## is. Note, however, that you can't actually measure what ##\delta \omega## is with the ruler; you're only getting a quantity that's proportional to ##\delta \omega##. If you don't see why that's the case, consider the units of ##\delta \omega##. So part of your task is to figure out how ##\delta \omega## is related to the linear distance between the points on the screen.

    Do you see what you would measure on the diffraction pattern to figure out what ##a## is?
     
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