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I'm experimenting with the one slit (and eventually two slit) light diffraction phenomenon.

The set up is simple (excuse the clutter):

Set up

The laser is a $532\ \mathrm{nm}, 1\ \mathrm{mW}$ device, placed about $10\ \ \mathrm{cm}$ from the vertical double slit, which is made from safety razors. The latter is about $166\ \mathrm{cm}$ from a wall and the slit is estimated to be $0.1 - 0.2\ \mathrm{mm}$ wide.

Here a good photo from a single slit run:

Single slit

The $10^{th}$ minimum is about $6.5\ \mathrm{cm}$ from the centre maximum.

I'm fairly pleased with that result but rather puzzled as to the unusual shape of the maxima, which the literature invariably shows as oblong ovals.

I've since tightened things up by making the slit more vertical and the laser beam more horizontal, as well as some other minor modification but to no avail. Another, this one red, laser gives the same result/problem.

Has anybody encountered this or have a possible explanation for these strangely formed maxima?

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There are two items to address here:

  1. Your expectation of oval shaped maxima is not true for general apertures. In particular, a rectangular shaped aperture yields maxima with a similar shape to yours.
    Rectangular aperture.
  2. The more intriguing property of your diffraction pattern is the top-bottom asymmetry. This means that the sides of your slit are not completely parallel. I expect that your slit is narrower at the bottom than at the top, because by the uncertainty principle for Fourier transforms a narrow slit will lead to a wide diffraction pattern.
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  • $\begingroup$ Yly, how about to mention that this perfect image is a computer simulation? $\endgroup$ – HolgerFiedler Feb 4 '18 at 14:56
  • $\begingroup$ Thanks, Yly. I'll experiment a bit with that. The sides seem pretty parallel to me but I'll verify that more closely. $\endgroup$ – Gert Feb 4 '18 at 15:24

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