Investigation 15C: Interference
Virtually all waves in the environment are composed of many frequencies at the same time, including sound waves and light waves. In physics, the term interference describes the addition of multiple waves that interfere with each other to produce a complex, multi-frequency wave that is the sum of many single-frequency waves.
Part 1: Constructive and destructive interference
1. Select the Analyze Waveform tab. Turn on the green sine wave. Create a wave that has a frequency of 50 Hz. Adjust the amplitude and time axis until you can see about 5 full cycles.
2. Add a second (red) wave of the same frequency and set the display to show the sum (interference) of the two waves.
3. Adjust the phase and amplitude of the individual waves until they interfere to make the largest possible or the smallest possible sum.
Questions
a. In your own words, describe what the phase and amplitude variables do.
b. Describe how you can create constructive interference, or a sum that is larger than either component wave.
c. Describe how you can create destructive interference (or a sum that is smaller than either component wave) by varying either the amplitude of the second (red) wave or its phase.
Part 2: Superposition principle
1. Create a wave that has a frequency of 50 Hz and adjust the amplitude and the time axis until you can see around 5 full cycles.
2. Add a second wave of a different frequency and set the display to show the sum (interference) of both waves.
3. Vary the frequency difference between the two waves and observe the waveform on both a short time scale (3-5 periods) and a long time scale (1 second or more).
Questions
a. In your own words, describe the difference in appearance of a wave with a single frequency compared to a wave with more than one frequency.
b. Describe how the amplitude changes over time when two waves interfere that differ in frequency by a small amount, such as 2 Hz out of 100 Hz.
c. In your own words, what does the superposition principle mean?
d. Write a multiple-choice question that tests a student’s understanding of the superposition principle.
1