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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.

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?

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/

//www.youtube.com/watch?v=TWu4U-ngMjkI’ve been teaching microwave polarisation wrong! - A Level PhysicsQuoteSo it turns out the way I've been teaching microwave polarisation is wrong!! Well, it's not so much wrong, it's the fact that the 'picket fence' analogy for polarisation isn't what it first seems. Where the picket fence only allows vertically polarised light through, a corresponding polarising filter only allows horizontally polarised light through! Watch this video for more explanation.

So it turns out the way I've been teaching microwave polarisation is wrong!! Well, it's not so much wrong, it's the fact that the 'picket fence' analogy for polarisation isn't what it first seems. Where the picket fence only allows vertically polarised light through, a corresponding polarising filter only allows horizontally polarised light through! Watch this video for more explanation.

These results will have profound impact on our understanding of diffraction and interference of light.

This will be the first step to explain a kind of physical phenomenon which has baffled most people like double slit experiment.

Some said it's mind boggling and defies logic, some others even said that it shows that reality doesn't exist.

I don't think the most people are baffled by the double slit experiment.

I think those might just be the crazy people.

https://plus.maths.org/content/physics-minute-double-slit-experiment-0One of the most famous experiments in physics is the double slit experiment. It demonstrates, with unparalleled strangeness, that little particles of matter have something of a wave about them, and suggests that the very act of observing a particle has a dramatic effect on its behaviour.

Most discussions of double-slit experiments with particles refer to Feynman's quote in his lectures: “We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics.

There will be some variations of my experiment, but here's the main scenario. A laser pointer is used as the light source to provide monochromatic coherent light. In front of it is a linear polarizer oriented diagonally 45° to the right from vertical axis. The light beam then hit a single slit aperture made of a pair of linear polarizers, where the "conductors" are oriented vertically. The light beam is then projected to a wall, where a single slit diffraction-interference pattern can be seen.Another linear polarizer is then inserted between the single slit aperture and the wall. When it's oriented vertically, only the central point is bright, while the fringes disappear. On the other hand, when it's oriented perpendicular to the first polarizer, the center spot gets much dimmer, while the fringes are still visible, although its intensity is also reduced.

All of those explanations would need to be revised.

https://en.wikipedia.org/wiki/Diffraction#Single-slit_diffractionA long slit of infinitesimal width which is illuminated by light diffracts the light into a series of circular waves and the wavefront which emerges from the slit is a cylindrical wave of uniform intensity, in accordance with Huygens–Fresnel principle.An illuminated slit that is wider than a wavelength produces interference effects in the space downstream of the slit. Assuming that the slit behaves as though it has a large number of point sources spaced evenly across the width of the slit interference effects can be calculated. The analysis of this system is simplified if we consider light of a single wavelength. If the incident light is coherent, these sources all have the same phase. Light incident at a given point in the space downstream of the slit is made up of contributions from each of these point sources and if the relative phases of these contributions vary by 2π or more, we may expect to find minima and maxima in the diffracted light. Such phase differences are caused by differences in the path lengths over which contributing rays reach the point from the slit.We can find the angle at which a first minimum is obtained in the diffracted light by the following reasoning. The light from a source located at the top edge of the slit interferes destructively with a source located at the middle of the slit, when the path difference between them is equal to λ/2. Similarly, the source just below the top of the slit will interfere destructively with the source located just below the middle of the slit at the same angle. We can continue this reasoning along the entire height of the slit to conclude that the condition for destructive interference for the entire slit is the same as the condition for destructive interference between two narrow slits a distance apart that is half the width of the slit.

https://en.wikipedia.org/wiki/Diffraction_from_slits#General_diffractionBecause diffraction is the result of addition of all waves (of given wavelength) along all unobstructed paths, the usual procedure is to consider the contribution of an infinitesimally small neighborhood around a certain path (this contribution is usually called a wavelet) and then integrate over all paths (= add all wavelets) from the source to the detector (or given point on a screen).Thus in order to determine the pattern produced by diffraction, the phase and the amplitude of each of the wavelets is calculated. That is, at each point in space we must determine the distance to each of the simple sources on the incoming wavefront. If the distance to each of the simple sources differs by an integer number of wavelengths, all the wavelets will be in phase, resulting in constructive interference. If the distance to each source is an integer plus one half of a wavelength, there will be complete destructive interference. Usually, it is sufficient to determine these minima and maxima to explain the observed diffraction effects.The simplest descriptions of diffraction are those in which the situation can be reduced to a two-dimensional problem. For water waves, this is already the case, as water waves propagate only on the surface of the water. For light, we can often neglect one dimension if the diffracting object extends in that direction over a distance far greater than the wavelength. In the case of light shining through small circular holes we will have to take into account the full three-dimensional nature of the problem.Several qualitative observations can be made of diffraction in general:The angular spacing of the features in the diffraction pattern is inversely proportional to the dimensions of the object causing the diffraction. In other words: the smaller the diffracting object, the wider the resulting diffraction pattern, and vice versa. (More precisely, this is true of the sines of the angles.)The diffraction angles are invariant under scaling; that is, they depend only on the ratio of the wavelength to the size of the diffracting object.When the diffracting object has a periodic structure, for example in a diffraction grating, the features generally become sharper. The fourth figure, for example, shows a comparison of a double-slit pattern with a pattern formed by five slits, both sets of slits having the same spacing between the center of one slit and the next.https://en.wikipedia.org/wiki/Diffraction_from_slits#Single_slit

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslit.htmlThe diffraction pattern at the right is taken with a helium-neon laser and a narrow single slit. The use of the laser makes it easy to meet the requirements of Fraunhofer diffraction. With a general light source, it is possible to meet the Fraunhofer requirements with the use of a pair of lenses.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinint.html#c1Under the Fraunhofer conditions, the wave arrives at the single slit as a plane wave. Divided into segments, each of which can be regarded as a point source, the amplitudes of the segments will have a constant phase displacement from each other, and will form segments of a circular arc when added as vectors. In this way, the single slit intensity can be constructed.

https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/University_Physics_III_-_Optics_and_Modern_Physics_(OpenStax)/04%3A_Diffraction/4.02%3A_Single-Slit_DiffractionFigure 4.2.3 : Light passing through a single slit is diffracted in all directions and may interfere constructively or destructively, depending on the angle. The difference in path length for rays from either side of the slit is seen to be a sinθ .Here, the light arrives at the slit, illuminating it uniformly and is in phase across its width. We then consider light propagating onwards from different parts of the same slit. According to Huygens’s principle, every part of the wave front in the slit emits wavelets, as we discussed in The Nature of Light. These are like rays that start out in phase and head in all directions. (Each ray is perpendicular to the wave front of a wavelet.) Assuming the screen is very far away compared with the size of the slit, rays heading toward a common destination are nearly parallel. When they travel straight ahead, as in part (a) of the figure, they remain in phase, and we observe a central maximum. However, when rays travel at an angle θ relative to the original direction of the beam, each ray travels a different distance to a common location, and they can arrive in or out of phase. In part (b), the ray from the bottom travels a distance of one wavelength λ farther than the ray from the top. Thus, a ray from the center travels a distance λ/2 less than the one at the bottom edge of the slit, arrives out of phase, and interferes destructively. A ray from slightly above the center and one from slightly above the bottom also cancel one another. In fact, each ray from the slit interferes destructively with another ray. In other words, a pair-wise cancellation of all rays results in a dark minimum in intensity at this angle. By symmetry, another minimum occurs at the same angle to the right of the incident direction (toward the bottom of the figure) of the light.At the larger angle shown in part (c), the path lengths differ by 3λ/2 for rays from the top and bottom of the slit. One ray travels a distance λ different from the ray from the bottom and arrives in phase, interfering constructively. Two rays, each from slightly above those two, also add constructively. Most rays from the slit have another ray to interfere with constructively, and a maximum in intensity occurs at this angle. However, not all rays interfere constructively for this situation, so the maximum is not as intense as the central maximum. Finally, in part (d), the angle shown is large enough to produce a second minimum. As seen in the figure, the difference in path length for rays from either side of the slit is asinθ , and we see that a destructive minimum is obtained when this distance is an integral multiple of the wavelength.Thus, to obtain destructive interference for a single slit,a sinθ=mλwherem=±1,±2,±3,... ,a is the slit width,λ is the light’s wavelength,θ is the angle relative to the original direction of the light, andm is the order of the minimum.Figure 4.2.3 : A graph of single-slit diffraction intensity showing the central maximum to be wider and much more intense than those to the sides. In fact, the central maximum is six times higher than shown here.

Here they are.