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I've just done a double-slit experiment in our school club using a simple kit with a pair of razor blades and a laser pointer. We put the razor blades together, cut a double slit in a clear film, and shot the laser pointer through the slits. The experiment worked out fine. Just as we expected, the laser pointer, which projected a single dot before, showed a line of small dots, obviously the result of the wave-particle duality. and we tested it on the uncut clear film, just to make sure the pattern wasn't a result of light scatter.

However, just for fun, we cut a single slit in another clear film, and shot the laser pointer again. Strangly, even though it wasn't a double slit, a double slit pattern still appeared.

Any ideas on how this can be explained?

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    $\begingroup$ Likely single-slit diffraction. Do you have a photo? $\endgroup$
    – Puk
    Dec 26, 2023 at 7:59
  • $\begingroup$ Or a sketch with a comparison of the double slit pattern. You may have noticed that the intensity of the double slit pattern was modulated by a single slit diffraction pattern. Related/duplicate - How can a single slit diffraction produce an interference pattern? $\endgroup$
    – Farcher
    Dec 26, 2023 at 9:03
  • $\begingroup$ In this experiment unless you use single photon detection there is no obvious wave particle duality. It is just covered by the wave model. $\endgroup$
    – lalala
    Dec 27, 2023 at 8:05

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This phenomenon is known as single-slit interference and can be explained several different ways. Fringe patterns are not exclusive to single and double slits; they also form behind triple and multiple slits experiments, including diffraction gratings. Even a single edge can generate a diffraction pattern. The following illustration shows examples of these patterns: enter image description here

To understand the single-slit interference pattern, consider the edges of the slit. The edges act as sources for photons as they diffract and scatter. When numerous photons travel to random points on the screen from these sources, they form a destructive pattern wherever the path differences to the screen are half a wavelength off.

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