Physical Double Slit Model

My favorite way to teach about the double slit experiment is to use physical model using transparencies that I learned from one of the Perimeter Institute's amazing resources. In this model, students manipulate two transverse wave forms to determine the locations of constructive and destructive interference.

In this post, I'm going to share the version that I use in my classroom

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Files

Physical Double Slit Model (pdf)
File Size:	508 kb
File Type:	pdf

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Physical Double Slit Model (editable)
File Size:	34 kb
File Type:	docx

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Wave Form Transparencies (pdf)
File Size:	210 kb
File Type:	pdf

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Wave Form Transparencies (pdf)
File Size:	504 kb
File Type:	pptx

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Materials

• Scissors
• Drinking straw
• Clear tape
• Sewing pins
• 1 ft^2 piece of carboard/formboard
• Ruler

To Print

• Waveform Transparencies – 2 of each size per set up – printed on a transparency film
• Double Slit Experiment – 1 per set up – printed on a standard sheet of printer paper

Wave Transparency Assembly

1. Cut apart the individual “boxes” found on the waveform transparencies sheet
2. For each of the transverse sine waves, cut a piece of a drinking straw the same length as the height of the wave (the shorter dimension) and attach it to the end of the transparency with small piece of clear tape

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Note: It is most fun and meaningful to be the one actually doing this activity, so it is recommended to make groups as small as possible (ideally working in pairs) so that there isn’t a lack of materials to interact with

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Investigation

1. Set up the physical double slit model by placing the “Double Slit Experiment” printout on top of the piece of cardboard or foamboard.
2. Pick out two identical transverse waveform transparencies and, using sewing pins, attach them to the print out by poking the pins through the straws into the formboard at the locations marked on the printout. In doing this step, you must ensure that the waves are “coherent” by flipping the straws until each wave starts with a wave crest or wave trough.
3. Pinching the transparencies together across the page on the solid line, find the position on the line where both waveforms would meet and constructively interfere. This will look like two transparencies perfectly overlaid with crests matching crests and troughs matching troughs. Mark this location with an “X” on the line.
4. Find any other locations along the solid line that would experience constructive interference and mark an “X” in these locations as well.
5. Overlap the transparencies in a location halfway between two “X”es. Describe what the wave combination looks like at this location.
6. With the small angle approximation, the double slit formula simplifies to s = Dλ/d, where s = fringe spacing, D = distance between slits and screen, d = distance between slits, and λ = wavelength.
7. Using a ruler, measure and annotate the distances for s, D, and d on the “Double Slit Experiment” handout
8. Use the measurements to calculate the wavelength of the wave.
9. With the ruler measure the actual wavelength of the waveform transparency used and calculate the percent error of your calculated approximation
10. Complete steps 2-9 for at least 1 other set of transverse transparencies

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Large Scale

My favorite spin that I've done on this activity is to create a large-scale version of the set up so that I can demonstrate in a classroom sized apparatus. I made this by using toilet paper rolls in the place of straws and packing tape that I carefully doubled up to eliminate the adhesive. I then printed out a sine wave of the desired size and traced it a couple of wavelengths at a time with a permanent marker.

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