A double-slit experiment with 3-cm microwaves will give you the basic understanding of how double-slit experiments are conducted in sophisticated quantum research that we will discuss in chapters 7 & 8. Just as so many of the other specialized particle detectors, the Gunnplexer receiver does not produce an image, so the interference pattern needs to be measured one point in space at a time. Observing the double-slit interference with the Gunnplexers requires the receiver to be scanned along the plane where the interference pattern is formed.
You will need to build a goniometer stand to conduct this experiment. The stand has two guide rails joined by a pivoting joint, with a protractor to measure the angle between the arms. We built ours (Figure 17) from 1-in. × 1-in. cross-section, T-slotted aluminum extrusions made by 80/20, Inc. We joined two 60-cm-long extrusions using a digital protractor made by iGaging. These are commonly sold by wood-working tool suppliers to enable accurate setting of table-saw blade and fence angles. We built two Gunnplexer unit holders from 80/20, Inc. L-brackets and 4-hole plates. We tapped a 1/4-in. hole in the back of the boxes we used to enclose the Gunnplexers, and used a knob screw to hold the units at any angle we require. A stick-on metric tape and two stick-on protractor dials complete our goniometer mount.
Figure 17 Our homemade goniometer stand is built from two pieces of 80/20, Inc. aluminum T-slot profile joined by a digital protractor. (a) Stick-on rulers and protractor dials make it easy to position the Gunnplexer units. (b) A folded sheet-metal holder with magnetic-tape backing supports slides right over the pivot point.
Make a slide holder out of sheet metal or a thin metal sheet that you can easily bend. Place a strip of adhesive-backed magnetic tape where the various slits and slides will be mounted. The plane for slide mounting should be centered on the protractor’s axis. Cut two double-slit slides out of sheet metal (or any other metal that sticks to a magnet). Make one spacer have a 6-cm-wide slit spacer, and the other a 9-cm-wide slit spacer. Make the slits 1.5-cm wide. Use a wooden dowel to keep the slotted slide straight.
Scan the receiver around the double-slit slide as shown in Figure 18a. You should find that the location of interference fringes as a function of the angle of detection is dependent only on the wavelength λ, and the center-to-center distance between the slits s. As the goniometer arms are moved, the received signal should peak at integer multiples of s(sin θ) = λ, as shown in Figure 18b.
Figure 18 Many experiments to observe the quantum behavior of particles use a nonimaging detector mounted on a goniometer stand to measure the interference pattern. (a) Measuring the double-slit interference with microwaves is an excellent way to prepare for more advanced experiments. (b) With this geometry, peaks in the interference pattern are found at integer multiples of s(sin θ) = λ.
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