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OT: Double slit experiment

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  • OT: Double slit experiment

    Has anyone actually tried the double slit experiment? I tried it recently with very basic materials. Black construction paper with both 1 and 2 slits, and a simple LED light source. With one slit, I just see one band of light passing through the slit and projecting onto a wall and with two slits, I see two bands projecting onto the wall... I don't see any interference patterns unless they are simply too faint to see with the naked eye. Maybe my slits were too close to each other or too far away from each other, etc, but I see what you would expect to see with partials. Oh well

  • #2
    I thought it had to be a laser? Otherwise the patterns are too diffuse to see on the wall?

    I did it in high school but I forget the specifics now.

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    • #3
      Diffraction effects rely on the slit being some lower number of wavelengths wide. So the slit needs to be DARN fine. And any second slit needs to be very close to the first. I can't see doing that with slits in construction paper done with basic scissors.

      I recall back in Physics 12 that we did diffraction labs that used a rig that held two knife edges which we would carefully bring together with a little screw setup. The final slit to see a good clear pattern was not visible to our eyes unless we held the light behind it or held it up to the sun coming in the room when the drapes were pulled back again at the end of the lab.
      Chilliwack BC, Canada

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      • #4
        An LED does not have adequate coherence, even if you make high quality slits.

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        • #5
          Yea, I was just using a LED flashlight and two 2" x 1/8" slits on black construction paper about 1" apart from each other projecting it onto my wall with a distance of about 1 foot from the light source and the construction paper and about 2 feet of distance between the paper and the wall being projected on. I'd really like to think we're inside of a massively complex simulation and one-day we'll figure it all out and be able to interface directly with reality and be able to engineer environments on a universe scale..

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          • #6
            You will need a coherent light source like a laser. The slits should be sharp, straight edges. I don't think cutting construction paper will give an edge sharp enough, but razor blades should work for a simple experiment. Try bringing just two razor blades together as a single slit the thickness of a piece of paper apart, with a small laser pointer as a source. You will probable need to have the lights out to see the fringe pattern.

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            • #7
              We did this experiment in high school physics. If I remember correctly we put black paint on a microscope slide, placed 2 razor blades side by side and drew 2 scratches in the paint. Interference patterns were very clear.

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              • #8
                Originally posted by Erich View Post
                We did this experiment in high school physics. If I remember correctly we put black paint on a microscope slide, placed 2 razor blades side by side and drew 2 scratches in the paint. Interference patterns were very clear.
                What did you use as a light source and how far away did you place the light source from the slide and the slide from the target?

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                • #9
                  I did this as a kid. I remember pulling thin copper wire past its yield limit to get really straight wire to make the two slits. Razor blades for the sides. I'm sure I didn't have access to any single wavelength source, unless maybe I had some kind of neon light, but I still got results. I remember mounting my microscope horizontally to view the interference pattern. All the measurements worked out.
                  Richard - SW London, UK, EU.

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                  • #10
                    A friend of mine did it recently. It works. Reality is a hard hard place.

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                    • #11
                      The real beauty of that experiment is if you reduce the intensity of the light to the point where only a single photon passes through the slits at a time, the interference pattern is still built up over time. You need to use film to record the strikes of the individual photons and you have to run it for many hours or even days before developing the film.

                      But a single photon can only pass through one of the slits. Yet due to it's dual particle/wave nature, it "knows" about the other slit and it behaves as if it interfered with another photon that passed through that other one. You don't get what you would expect as the superimposition of the pattern for two individual slits, which would not be an interference pattern. You get the actual interference pattern, just as happens with more intense levels of light so the light from the two slits can interfere with each other. In effect, a single photon can pass through both slits and interfere with itself.

                      As one physicist said, probably paraphrased a bit, the world in not just stranger than we think it is, it is stranger than we CAN think it is.
                      Paul A.
                      SE Texas

                      Make it fit.
                      You can't win and there IS a penalty for trying!

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                      • #12
                        We never had lasers back in high school. But we still did the experiment.

                        I may also have mixed up diffraction with interference. The deal we did with the slit was a single slit so that's diffraction. We did a different experiment for interference but it wasn't the slide with black paint and razor cuts.
                        Chilliwack BC, Canada

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                        • #13
                          Originally posted by Paul Alciatore View Post
                          The real beauty of that experiment is if you reduce the intensity of the light to the point where only a single photon passes through the slits at a time, .
                          we've all read of the experiment before (or assume so) and had our brains hurt as a result, where I get confused is how does one emit a single photon
                          in Toronto Ontario - where are you?

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                          • #14
                            Originally posted by Mcgyver View Post
                            we've all read of the experiment before (or assume so) and had our brains hurt as a result, where I get confused is how does one emit a single photon
                            With some difficulty. The first single-photon source was made in 1974. Some of the laser guys at work with built a single photon source for an experiment. As far as I know, it's still in the basement.

                            I did the dual slit experiment with electrons and a Mollenstedt interferometer instead of light and slits when I was in undergrad. Macona here on the forum actually sent me some vacuum parts to accomplish it. The idea was to eventually de-focus my electron source to a point where I could be reasonably certain of single electrons propagating through the interferometer. The electrons were incident on an extremely sensitive phosphor coating; the coating and electron energy were chosen such that a single electron would yield enough light to be visible. I never made it that far, though. I was able to get the expected results with large numbers of electrons but wasn't able to attenuate my electron source down to single electron level.

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                            • #15
                              I wonder if photons can pass through a graphene lattice. If not, one could search through sheets of graphene until they find one with a single broken lattice and use that as a pin-hole for single photons to pass through -- or however many neighboring broken lattices are needed until a single photon can pass though at a time.

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