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Optics Q : Diffraction Pattern from a Ronchi Ruling

  1. Dec 1, 2009 #1
    Optics Q!!!: Diffraction Pattern from a Ronchi Ruling

    Hey everyone,

    allow me to explain the experiment I'm working on before I get into my question. The experiment has light passing through a slit at the focal point of a lens. Since the light is at the focal point, theoretically it should become collimated as it continues past the lens. The light then passes through a ronchi ruling, and through another lens. What I've done is put a CCD camera at the Fourier plane (focal point of the second lens) and taken an image. My problem is in fitting to the intensity.

    Theoretically, the function I would be using to fit would be a convolution of the Fourier transform of a single square wave and a dirac comb and then squaring it. The Fourier transform of a single square wave is a sinc, and the transform of a dirac comb is approximately another dirac comb. The convolution of these gives me the electric field at the Fourier plane. Squaring obviously gives me something that is proportional to intensity.

    So anyway, the fit doesn't work. My professor told me that the 2 reasons for this are: the Ronchi ruling doesn't have an infinite number of gratings, the slit isn't exactly a 'point source'. I know how to account for the being a lack of gratings, but I'm not sure how to accommodate for the slit width. I was thinking, since the slit is at the focal point of the first lens, I could take the transform of that and somehow work it into my real image equation (original equation before transform).

    I hope that made sense
  2. jcsd
  3. Dec 1, 2009 #2

    Andy Resnick

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    Re: Optics Q!!!: Diffraction Pattern from a Ronchi Ruling

    What happens when you rotate the Ronchi ruling with respect to the slit orientation? That should give you some clues.

    Also, it sounds like you have some sort of 4-f system. Is that correct?
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