Dark spot in Fraunhofer diffraction

In summary, the conversation discusses the issue of dark spots appearing in the middle of patterns during Fraunhofer diffraction, despite not being predicted by the Fourier transform of the aperture. The conversation also mentions the possibility of Babinet's principle and the effect being stronger near the edges of the slits. However, it is clarified that near the slits would correspond to Fresnel diffraction and the specific slit or aperture geometry is a narrow slit approximately 100 microns wide and 2-3mm high. The cause of the dark spots remains unknown.
  • #1
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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  • #2
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?
 
  • #3
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.
 
  • #4
Weird, not sure what would cause that.
 
  • #5


Hi there,

Thank you for reaching out and sharing your observations. Dark spots in Fraunhofer diffraction can occur due to a phenomenon called diffraction interference. This happens when light waves from different parts of the aperture interfere with each other and create areas of constructive and destructive interference, resulting in dark and bright spots in the diffraction pattern.

One possible explanation for the dark spots in the middle of your patterns could be due to the presence of a secondary source of light, such as ambient light or stray reflections, that is interfering with the primary light source and creating interference patterns. This could also explain why the effect is stronger near the edges of the slits, as the edges may be more susceptible to stray reflections.

Another possibility could be that the aperture itself is not perfectly uniform or smooth, which can also cause diffraction interference and create unexpected dark spots in the pattern. I would suggest checking the quality and uniformity of your aperture to see if that could be a contributing factor.

I hope this helps and gives you some ideas to explore. Keep in mind that diffraction interference can be a complex phenomenon and may require further experimentation and analysis to fully understand. Best of luck with your research!
 

1. What is the cause of dark spots in Fraunhofer diffraction?

The dark spots in Fraunhofer diffraction are caused by destructive interference between the diffracted waves from neighboring points on the aperture. This occurs when the path difference between these waves is equal to a half-wavelength or an odd multiple of a half-wavelength.

2. How can dark spots be minimized in Fraunhofer diffraction?

To minimize dark spots in Fraunhofer diffraction, the aperture size can be increased, which will decrease the path difference between the diffracted waves and reduce the likelihood of destructive interference. Additionally, using a narrower wavelength of light can also help reduce the size and number of dark spots.

3. Can dark spots be completely eliminated in Fraunhofer diffraction?

No, it is not possible to completely eliminate dark spots in Fraunhofer diffraction. This is because diffraction is a fundamental property of light and occurs whenever light passes through an aperture or around an obstacle. However, the size and number of dark spots can be reduced by optimizing the experimental setup.

4. Do dark spots in Fraunhofer diffraction affect the overall intensity of the diffraction pattern?

Yes, the presence of dark spots in Fraunhofer diffraction does affect the overall intensity of the diffraction pattern. The destructive interference that causes the dark spots also results in a decrease in the intensity of the diffracted waves at those points. However, the overall intensity of the diffraction pattern can still be significant and useful for experiments and measurements.

5. Are dark spots in Fraunhofer diffraction only observed in visible light?

No, dark spots in Fraunhofer diffraction can be observed in any type of light, including visible, infrared, and ultraviolet. This phenomenon is a result of the wave nature of light and is not limited to any specific wavelength range.

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