Interference Lab

A. Interference Simulation

Part 1:

1. Go to https://phet.colorado.edu/en/simulation/wave-on-a-string

2. Click the play button to run the PHET

3. Select "Pulse", "Fixed End", on the top, "Normal" in the middle bottom, and move the slider for damping to "None" and the slider for "Tension" to the middle. Keep the amplitude at 0.75 cm, and the pulse width to 0.50 s

4. Click the green circle on the pulse device, and send a pulse down the string. Just as the first pulse reaches the fixed end, click again to send a second pulse down the string.

5. Watch the pulses colliding in the center and interfering. On one of the cycles, hit the pause button right before the pulses collide. Use the frame by frame button (right next to the pause/play button) to watch the overlapping of the waves.

6. What kind of interference is this? Constructive or destructive?

Part 2:

1. Click the "Restart" button.

2. On the top right, now click "Loose End"

3. Click the green circle on the pulse device, and send a pulse down the string. Just as the first pulse reaches the loose end, click again to send a second pulse down the string. Notice that every other collision is constructive, the other being destructive.

4. Watch the pulses colliding in the center and interfering. On one of the cycles where the colliding pulses are in the same direction, hit the pause button right before the pulses collide. Use the frame by frame button (right next to the pause/play button) to watch the overlapping of the waves.

5. What kind of interference is this? Constructive or destructive?

B. Interference Game (You can't do this at home probably - which is sad - just watch these videos though - they are pretty cool)

1. Watch this video: https://www.youtube.com/watch?v=ypcX1LdmMPM

2. Watch this video: https://www.youtube.com/watch?v=QjukfQttKAQ (Actually watch it)

3. Stretch a big slinky out 20 feet in the hall. (Or a small slinky 10 feet) Center the cups so that you can coordinate with each other, and send the pulses at the same time.

5. Set up the cups like this: Send two pulses that interfere, and knock the end cups over but don't touch the center cups. What kind of interference is this?

C. Two source PHET

1. Go to https://phet.colorado.edu/en/simulation/wave-interference

2. Run the PHET and click twice on the one called "Interference"

3. Click the speaker icon on the bottom middle of the toolbar on the right to make it generate sound waves

4. Click each of the green dots on the speakers to turn them on

5. Grab the sample tool (Looks like an "Etch a Sketch" with two paddles on it) out of the top of the toolbar, and put it below the toolbar.

6. Move one of the sample paddles to different spots in the simulation. Find places where the sound is loud (Constructive interference), find places where the sound is quiet (Destructive interference). Notice that you can see in the pattern that these quiet spots radiate away from the two sources. Put the tool away

7. This is how sound cancellation technology works - a microprocessor sends sound waves that destructively interfere with ambient noise. (Like your parents calling you to dinner)

8. Change the frequency to "max" - What happens to the spacing of the quiet bands we saw above?