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?