What causes a cross shaped diffraction pattern with two sets of double slits?

Question

I am a retired computer programmer very interested in physics and in particular wanting to find out about Young's double slit experiment at home. I have used a small pen laser and shone the light through double slits formed from very fine wire and black masking tape. This gives the classical bright central dot and a series of bands stretching out on either side. However, if I incorporate a second double slit between the first and the screen and this one's orientation is at right angles to the first I observe a cross-shaped pattern with dark bars running vertically and horizontally throughout it. This is particularly pronounced in the bright spot at the centre. Both sets of slits are longer parallel to the slits than across, but the cross pattern still seems to persist when the second one has this reduced so that both vertical and horizontal lengths are the same. My question is what is causing the vertical and horizontal banding? I thought that a second slit would simply form a new fainter pattern which was entirely independent of the first one.

Answer 1

I'm not exactly sure what you saw, but for 2 orthogonal double slits, a pattern like the following is expected:

(laser wavelength 600nm, diameter 3mm, slits distance 0.5mm, slit width 0.1mm, distance of the two double slits 5mm, image at 50cm)

The single double slit has already an inner structure (see the upper left image). If you add another double slit, it will add the same structure in the other direction, and you are right, it is basically independent from the first direction. That combination will give you a cross pattern.

With a commercial double slit, you can usually see (or at least guess) the darker regions in the first maximum with the naked eye (the axis here are in mm), while the cross pattern is actually easier to spot. Since you built your own double slits, your image might not be clear enough to see the inner structure in the first, just the second image.