Place an IF-508 diffraction mosaic slide at a distance of 1 m away from a sheet of white paper placed on the wall, as shown in Figure 4a. Using laser pointers of different wavelengths, measure the fringe separations of the interference pattern produced when the beam passes through the double-slit separations of s = 4.5, 5.8, 7.5, and 10 μm. How well do your measurements agree with d = λr/s? How does the wavelength of the laser affect your ability to measure the fringe distance? Explain.
2. Replace the IF-508 in the prior setup by a human hair. Using laser pointers of different wavelengths, measure the fringe separations of the interference pattern produced when the beam passes through the hair. Calculate the thickness of the hair. Try thinner and thicker hairs. If available, try a spider web filament. How does the thickness of the hair affect the interference pattern? What combination of wavelength and hair thickness gives the widest fringe separation? Explain.
3. Set up the Gunnplexer units as shown in Figure 16. Measure the received signal, and then reposition the Gunnplexers to increase the distance between the units by 5 cm. Do this for distances between 20 and 60 cm (or more if your setup allows it). Do your data support the idea that the Gunnplexer’s detector is sensitive to the intensity I of the electric field, which falls in proportion to the inverse of the square of the distance (1/r2) between the transmitter and the receiver? Explain.
4. Place the receiver and transmitter on a goniometer stand so that they face each other at a distance of around 50 cm, with the pivot point exactly midway between them. Take a reading of the received signal strength, and then change the angle between the arms by 5°. Sweep the arm to obtain measurements between −90° and +90°. How tight is the microwave beam? How well does it match a Gaussian distribution? Explain.
5. Place the receiver and transmitter on a stand so that they face each other at a distance of around 50 cm. Take a reading of received signal strength, and then rotate the receiver to change its polarization angle by 5°. Continue to rotate the receiver to obtain measurements between −90° and +90°. How does relative polarization affect the intensity of the detected signal? Does the intensity as a function of polarization follow a smooth curve? Explain.
6. Set up the Gunnplexer units as shown in Figure 16. Measure the received signal when r = 0 and then slowly move the Gunnplexers away from one another. Record the distances at which the signal peaks and dips out to around 20 cm. Calculate wavelength by measuring the distance between successive peaks (or successive dips), which should equal λ/2. What is the frequency f[Hz] = c/λ[m] = 3 × 108[m/s]/λ[m] at which your Gunnplexer is transmitting? How well does the calculation of frequency based on the average of your measurements compare to the specified frequency for your Gunnplexer? Explain.
7. Place the receiver and transmitter on a stand so that they face each other at a distance of around 50 cm. Rotate the receiver 90° away from the transmitter’s axis of polarization. The signal at the receiver should drop close to zero. Insert a polarizer at 0°, 45°, and 90° referenced to the transmitter’s polarization. How can inserting the polarizer at 45° increase the signal level at the detector?
8. Take three pieces of linear-polarizing film and look at the reflection of low-angle sunlight from a water surface with each one. Rotate each piece of film until reflections are minimized. Mark the vertical axis on each slide as its axis of polarization. Try the Figure 15 experiment with your labeled pieces of polarized film. Do your results agree with Figure 15? How does rotating one of the pieces of film in Figure 15a and Figure 15c change the amount of transmitted light? Explain.
9. On a clear day, look at different portions of the sky with a piece of polarizing film. Are any areas polarized? What is the polarization of light reflected from pavement? Explain your results.
10. Look at a pair of polarized sunglasses through one of your labeled pieces of polarizing film. What is the axis of polarization of the lenses in the sunglasses? Check out other sunglasses. Are they all polarized in the same orientation? Why do you think sunglasses are polarized in this way?
11. Polarizing film acts as a chemical version of the wire grid. Instead of long, thin wires it uses long, thin polyvinyl alcohol molecules that contain many iodine atoms. These long, straight molecules are aligned almost perfectly parallel to one another, with electrical conductivity provided by the iodine atoms. Examine a polarizing sheet very carefully. Can you see a pattern that resembles a wire grid? Would it be visible under a light microscope? Explain.
12. Place the receiver and transmitter on a goniometer stand so that they face each other at a distance of around 50 cm, with the pivot point exactly midway between the receiver and transmitter. Place a double-slit slide with s = 75 mm over the pivot point, as shown in Figure 18. Take a reading of received signal strength at θ = 0°, and then change the angle between the arms by 5°. Sweep the arm to obtain measurements between −90° and +90°. Repeat, using a double-slit slide with s = 105 mm. Plot your data, and identify the angles at which the peaks and dips of the interference pattern occur. Are successive peaks (or successive dips) separated according to sin θ = λ/s? Explain.
13. Connect the Gunnplexer’s detector diode output (mixer output) to an audio amplifier. Point the Gunnplexer at a passing car and pay attention to the whooshing sound that you will hear. How does the Doppler sound produced by the Gunnplexer from an approaching car compare to the sound produced when the Gunnplexer is pointed at a receding car? Explain.
* A double-slit slide can be made at home by coating a piece of clear glass with dark paint and then scoring the double slit with two narrowly-spaced razor blades. The best way to produce a quality slide is to apply two parallel strips of adhesive tape, leaving a 1/2-in. band of glass uncovered. A large drop of paint applied toward one end of the bare strip is then spread with a razor blade along the strip to deposit a very smooth, constant-thickness layer of paint. Two brand-new razor blades should then be stacked, using a paper spacer between them. The two parallel blades should then be used with a brisk motion to score a pair of lines across the dry paint. The result should be two hairline transparent slits separated by an extremely thin line of paint.
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