[hamdani yusuf (OP)]

And here's another experiment related to the previous one. This time we use double needle.

It demonstrates the result of double thick wire experiment, or double needle experiment, which produce diffraction-interference pattern like standard double slit experiment, if the gap between the needles are narrow.

[bamgstrom]

In the last video, are you looking at the proper part of the diffraction pattern? There should be a broad diffraction pattern from the wires as shown in your previous videos. The bright red spot at the center shows a good diffraction pattern as expected but there is a possibility that it could be caused by some irregularity in the laser itself. The diffraction pattern beyond the bright red dot should be easily visible at a much shorter distance from the laser source.

[Bored chemist]

The first picture is the direct laser spot when allowed to hit the wall without obstruction. Due to lens imperfection, some artefact is visible on the wall. To avoid complication, the laser is oriented so that the spots are aligned vertically, which is on the same axis as the obstructing needles that will be used.

You can improve the quality of the laser beam  which will solve that problem.
https://www.edmundoptics.co.uk/knowledge-center/application-notes/lasers/understanding-spatial-filters/

[hamdani yusuf (OP)]

In the last video, are you looking at the proper part of the diffraction pattern? There should be a broad diffraction pattern from the wires as shown in your previous videos. The bright red spot at the center shows a good diffraction pattern as expected but there is a possibility that it could be caused by some irregularity in the laser itself. The diffraction pattern beyond the bright red dot should be easily visible at a much shorter distance from the laser source.

Most online formulas describing double slit experiment only consider the distance between the center of the slits, while omitting the width of the slits. The cross sectional shapes of the slits/wires are mostly ignored. So, those are precisely what we need to investigate further.
What do you expect to see if I use a better laser pointer?

[bamgstrom]

Most online formulas describing double slit experiment only consider the distance between the center of the slits, while omitting the width of the slits. The cross sectional shapes of the slits/wires are mostly ignored. So, those are precisely what we need to investigate further.
What do you expect to see if I use a better laser pointer?

My poor quality laser shows a “bull’s eye” interference pattern due to what must be some internal interference even when there is nothing in the path to produce an interference. That is why I prefer to examine the parts of interference pattern that extend beyond the bright dot in the very center.

That is also why I suggest placing the screen closer to the laser so the entire diffraction pattern is visible and not just the center. You did that in the earlier videos but not in the one with the wires and long hallway.

A poor quality laser works well enough for masked experiments such as single or double slits, but for wide open experiments with wires, you can’t be certain where the interference is coming from by looking at the bright spot in the center. However it did work nicely with the nails. That may be because they are larger and block more of the stray light.

I don’t have a good quality laser for comparison.

[hamdani yusuf (OP)]

I've also done the experiment of double needles in series, and repeat the single needle experiment with various distance from the wall.  I got an unexpected result.

I have several laser pointers with different price tags. I think I'll try to use them for comparison.

[Bored chemist]

I don’t have a good quality laser for comparison.

You can improve the quality of the laser beam  which will solve that problem.
https://www.edmundoptics.co.uk/knowledge-center/application-notes/lasers/understanding-spatial-filters/