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I recently completed an experiment using white light through both a circular aperture and four-piece diffraction slide at the same time. I'm trying to qualitatively explain what I'm seeing, which I know are diffraction spikes, but I don't know what's caused them.

The light shines through the initial plate which has one central hole with smaller holes located vertically and horizontally. You can see them in the far right pictures. After passing through those, it hits the plate with four different diffraction gratings, shown on the left. The right four photos are the result of this.

I initially thought the diffraction spikes were created by the smaller holes in the first plate, as it seems like the same result you get for diffraction spikes when using a bladed aperture in a camera. For a camera with a 6 bladed aperture, the waves diffract perpendicular to the edges of the blades and create spikes near the vertices. This would also seem to be the case for reference holes in a + shape, which would create an X shaped bunch of spikes.

The only problem with this is that the same spikes are visible while looking through the smaller holes as well (if you look at the pictures on the right, you see the camera was angled so that the light was coming through a smaller hole, not the central large hole).

I also thought maybe the diffraction was caused by the tiny squares in the top diffraction gratings, but the same pattern comes from the bottom gratings which consist of randomly spaced circles, so now I'm out of ideas.

Sorry for writing so much, does anyone know what actually caused these spikes?

enter image description here

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If you reduce the intensity of the light, and perhaps increase the image capture time, the X shapes should resolve into a series of dots. The far-field diffraction pattern of a rectangular aperture appears as a bright central spot with four arms; a closer look finds dimmer spots between the arms.

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  • $\begingroup$ So you're saying it's just the + shaped aperture behind the diffraction grating that's causing these spikes? When we used a different diffraction grating that was made up of only lines, we got a circular pattern, which is why I was hesitant about saying it was just from the aperture. $\endgroup$
    – physkid
    Commented Mar 27, 2016 at 3:38
  • $\begingroup$ I may have misread the geometry involved; please elaborate the details of the setup and the results, and I'll reconsider. $\endgroup$
    – Peter Diehr
    Commented Mar 27, 2016 at 10:50
  • $\begingroup$ The light passes straight through an initial plate that contains a central circular aperture surrounded by smaller horizontal and vertical holes, which you can see in the left pictures. The central aperture is the largest, the holes make a + shape. Then, the light passes through the diffraction plate shown in the left picture. The plate contains four different gratings. The pictures on the right are what the light looks like through both the circular + shaped aperture and the diffraction gratings. There are four pictures on the right to correspond to the pattern we see through each grating. $\endgroup$
    – physkid
    Commented Mar 29, 2016 at 4:27
  • $\begingroup$ What are the sizes of the holes? How far apart are they? How many lines per cm on the diffraction plates? What type of camera is used? $\endgroup$
    – Peter Diehr
    Commented Mar 29, 2016 at 11:04
  • $\begingroup$ The large central hole is about 2mm, the smaller ones are about 0.5mm. Distance from central hole to first hole is 13mm, with 6.3mm between each small hole. There were no details given about the diffraction grating as it's qualitative analysis only and a smart phone camera was used. If the holes acted as a + shaped aperture, this should be the result $\endgroup$
    – physkid
    Commented Mar 29, 2016 at 14:15

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