Dark spot in Fraunhofer diffraction

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Dark spots observed in Fraunhofer diffraction patterns may not align with predictions from the Fourier transform of the aperture. The issue seems to be more pronounced when the incident beam is near the top or bottom edges of a narrow slit, approximately 100 microns wide and 2-3 mm high. This suggests a potential influence of near-field diffraction effects, despite the focus on Fraunhofer diffraction. Further clarification on the specific aperture geometry could help diagnose the cause of the unexpected dark spots. Understanding the interaction between the incident beam and the slit edges is crucial for resolving this phenomenon.
thepopasmurf
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Hi, I've been doing Fraunhofer diffraction and I have been getting dark spots in the middle if my patterns, not predicted by the Fourier transform of the aperture. I don't think it's babinets principle and effect is stronger near the edges of the slits. Any ideas? Thanks
 
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Not sure what would cause that. What do you mean by "near the edges of the slits"? Near the slits would correspond to Fresnel (near field) diffraction, not Fraunhofer diffraction. What is your specific slit or aperture geometry?
 
What do you mean by "near the edges of the slits"?

I mean when incident beam is near the top or bottom of the slit aperture.
The aperture in question is a narrow slit, around 100 microns wide (i think, that's off the top of my head) and perhaps 2mm or 3mm high.
 
Weird, not sure what would cause that.
 
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