Double slit experiment

Some time ago I read this and show this (The whole list here, especially the double slit experiments series) and this and this. Then I read some more and more.

While this is really amazing I wanted to test it myself. The following is an easy setup that anyone with a somehow programmable camera (i.e. you can change shutter speed) and a common laser pointer can do.

Results

Here are the results of 3 different experiments (you may need to click on the photos for better resolution):

Experiment 1:

  • One slit:
  • Two slits:

Experiment 2:

  • One slit:
  • Two slits:

Experiment 3:

  • One slit:
  • Two slits:

Amazing eh? 🙂

Do it yourself

You can test it yourself. For the experiment you’ll need:

  • Blu tak
  • Pencil leads
  • A laser pointer
  • Some batteries for the laser pointer
  • Black thick paper (e.g. black carton. I used a black folder)
  • A camera where you can change the shutter speed and (optionally) with optical zoom.

The setup will look like this:
Setup of experiment

Here are the steps:

  1. First make sure you have fresh batteries for the laser pointer. I used 3 AA batteries instead of 3 LR41. They should last much longer (just compare their size :-). Both of them (AA and LR41) are 1.5V.
  2. I used a paper clip to keep the button of the laser pointer pressed at all time. You’ll need to find a way to have the laser on without touching it.
  3. Get a small piece of the black paper and open a very small slit. Then put this in front of the laser and stick it at the bottom with the blu tak. This part ensures that only a small part of the laser beam will pass and that it will be very focused.
  4. Using blu-tak glue together 3 pencil leads as shown here:
    3 pencil leads, sticked together
    This will act as your double slit. The laser beam that passes through the one-slit paper should drop on those pencil leads and light two slits.
  5. Since light is tricky, you should make sure that no light passes outside of the two slits, so you will have to use two pieces of black paper to block light. See the setup picture for the idea. If you do it right, you should see the laser beam cut at the outside pencil leads.
  6. Put the camera directly in front of the laser beam. To do this you’ll have to first remove the pencil leads and aim at the beam. At this point you’ll have to make some adjustments to height of the camera and the laser. That’s why I used a book and a ruler bellow the laser pointer (moving it left-to-right allowed for smooth up-down movements of the laser beam).
  7. In order to adjust the camera you’ll need to become familiar with what moves what. For example, the beam is centered on the display by tilting the camera, while it is focused by moving the camera. Just do it yourself.
  8. Use as much zoom as possible and turn camera focus to infinity. Then adjust the shutter speed to appropriate values (I used values between 1/15 and 1/90 IIRC).
  9. From the first picture you’ll be able to see the distances that worked for me. That’s why I added the second ruler at the bottom of the image.

Here are two additional photos of the left and right sides of the setup:
Laser, connect to batteries and the single slit in front
Right side of setup: Camera, pencil leads

After setting up the experiment you’ll have to start testing it. First make sure that laser light passes between both slits and only between them. If you take a picture you should see something like the two-slit results (you may need to zoom inside the pictures to actually see it). Then move the black papers a bit to prevent light passing from one of the two slits and take another picture.

TATA! Now that you performed the experiment you may want to try and figure out why this happens. If you eventually figure it out you’ll have solved one great unsolved quantum mystery. So simple yet so tricky!

Explanation of the experiment results

The easiest result to explain is the result of experiment 2: The first photo shows what the camera captured when laser light was passing from one slit while the second show the result when light was passing from both slits. It is clear that there is some interference at the second case. In fact, allowing light to pass from both slits results in areas that are darker than when light was passing just from one slit (!). It is easy to see the two-slits because there are vertical lines. The height of each vertical line is no more than some millimeters.

Same thing for experiment 1 (click on the photos to actually see it): At first you may believe that this is a problem with focus, but the focus was fixed for both photos.

Experiment 3 is more eye-catching because it shows two different effects: The large light bands are the result of light diffraction while the smaller patterns are interference patterns. Diffraction is also visible with naked eye. Just put a piece of paper between the laser and the stencil leads and you will see the light bands. Unfortunately I could not get a photograph because they were not visible at the camera (i.e. the photos just showed a single spot and some artifacts. Test it yourself. It’s very impressing!)). Interference on the other hand is not visible with naked eye.

Let me end this post with a phrase from matrix: “Welcome to the real world”

4 Comments

  1. I was under the impression that the width of the slits and the distance between them should be of the same order as the wavelength of light being used. Red light is about 650 nm (0.00065 mm), but the width between your slits (at least) is much bigger.

    • I’m not a physicist so I can’t answer your question and be sure that what I’m saying is correct. I may only claim that from what I infer from the Wikipedia article, this is not a requirement. According to that article (assuming that I read it correctly), the distance F between the gaps is D*λ/B, where D is the distance of the slits with the camera, λ is the wavelength and B is the distance between the slits.

      In experiment #2, the two vertical lines have a distance of about 1mm and there are about 10 black lines in this distance. This gives: F=0.1mm, λ=650nm, B=1mm. This means that D=F*B/λ = 10^-4 * 10^-3 / 650 * 10^-9 ~=15cm.

      From the setup picture you can see that the distance of the slits and the camera’s CCD is about 6cm. Optical zoom is also involved and I’m not sure how this affects the result.

      This train of thought seems to match the results. But it is only a rough approach.

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