 PhysicsQuest

WAVE BEHAVIOR                          ACTIVITY II.

INTERFERENCE
PART 1. Wave Interference
PART 2. Principle of Superposition

PART 1. WAVE INTERFERENCE

What happens when two waves meet while they travel through the same medium? What effect will the meeting of the waves have upon the appearance of the medium? Will the two waves bounce off each other upon meeting or will the two waves pass through each other?

Wave Interference occurs when two waves meet while traveling along the same medium. To begin our exploration of wave interference, consider two pulses of the same amplitude traveling in different directions along the same medium.

1. Predict what will happen in the following situation. Sketch your prediction:
PART 2. PRINCIPLE OF SUPERPOSITION

To determine the resultant wave we can use the Principle of Superposition:

“When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.”

1. For each of the cases on questions 1, 3, 4 and 6 find the amplitude of the resultant wave if each square represents one cm.  Q1. _________     Q3. _________  Q4. _________     Q6. _________

2. To determine the precise shape of the medium at a given instant in time, the principle of superposition must be applied to several locations along the medium. In this example several locations have been labeled.  Use the principle of superposition for each location and mark the resultant amplitude with a dot.  Neatly connect the dots to find the resultant waveform.
3. Several positions along the medium are labeled with a letter. Categorize each labeled position along the medium as being a position where either constructive or destructive interference occurs.
CONCLUSION

1. Sound is a pressure wave that consists of compressions and rarefactions. Explain constructive interference of two sound waves in terms of pressure variations.

2. A friend tells you that the word destructive tends to imply that the energy as well as the form of the waves is destroyed. Do you agree with your friend? Explain your answer.              GO TO: ACTIVITY III  2. Click on the SIMULATION.  You can advance it slowly by using the cursor. Describe what you observe:

Does the behavior shown in the simulation agree with your prediction?

This type of interference is called Constructive Interference. Constructive Interference occurs when the wave amplitudes reinforce each other, building a wave of even greater amplitude.

3. What would happen if the pulses are inverted, i.e. a trough meets a trough?  Sketch the resultant wave:  4. Another type of interference occurs when the waves are out of phase i.e. a crest meets a trough. Sketch your prediction below: 5. Click STOP on the SIMULATION and select EQUAL Amplitude and OUT Phase. Describe what you observe:

Does the behavior shown in the simulation agree with your prediction?

This type of interference is called Destructive Interference. Destructive Interference occurs when the wave amplitudes oppose each other, resulting in waves of reduced amplitude.

6. Now suppose that the pulses have different amplitudes. Predict what would happen by drawing a sketch: 7. Click STOP on the SIMULATION and select UNEQUAL Amplitude. In this simulation the wave with larger amplitude is the red wave. How does the resultant wave compare to your prediction?   Constructive Interference:

Destructive Interference: