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I tried to run the double slit experiment at home for my kid with a cheap red laser pointer. I partially covered the laser with four pieces of electrical tape to make a tiny square hole. To my surprise that was enough to get a cross shaped interference pattern on my wall with a splatter of other dots around it.

covered laser hole setup cross interference cross interference zoom

When I covered the hole with a single hair strand I did get the nice classic new line of interference (perpendicular to the hair).

The "why do I get the cross beforehand" seems to be answered here (How can a single slit diffraction produce an interference pattern?) that unfortunately I only partially understand. Is there an easy way to fix this? Is my laser not producing "coherent enough" light and I need a different light source? Is the hole still too big (too small)? Would it help if I managed to place the slit further away?

Thanks!

interference with hair interference with hair marked

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    $\begingroup$ In my answer to the question I explain how to make a double slit. How can a double slit interference pattern be created without out going through a double slit. $\endgroup$
    – Farcher
    Dec 17, 2021 at 23:32
  • $\begingroup$ I’m unclear on what the problem is here. You want to demonstrate diffraction, but you’d rather it be double-slit diffraction because you understand it better than single-slit diffraction? $\endgroup$
    – Gilbert
    Dec 18, 2021 at 3:34
  • $\begingroup$ How was the hair orientated relative to the square hole? $\endgroup$
    – Farcher
    Dec 18, 2021 at 10:00

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I am sure that your laser pointer is fine for the job.

The cross is due to the fact that your hole is square.
Try making a six sided hole and see what you get - 3 lines inclined at $60^\circ$ to one another?

It is due to the diffraction of light and called a diffraction spike or starburst effect in photograph as shown below.

enter image description here

If you do not want the starburst then use a circular hole.

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