PHYS 1401 – General Physics I

The Properties of Waves

Leader: Reporter: _______

Skeptic: ___________________ Encourager: _________________________

Materials

Slinky

Laptop

Part 1a: Reflection and Interference of Waves – Java Simulations

The phenomena of reflection and interference can be difficult to observe with a slinky. In order to help you see the desired phenomena in a slinky, we will first observe it in the idealized situation of a computer simulation.

Interference

Open Internet Explorer and go to the URL www.austincc.edu/nrgpsc/phys1405/labs/wavesuper

Scroll down to the bottom of the page and click on the link labeled Animation 1a. When you click on the Play Button, two square wave pulses will move towards each other. You can stop the pulses by clicking on Pause, and move them one step at a time by clicking on Step >>.

1) Sketch the appearance of the pulses just before they overlap.

2) Sketch the appearance of the pulses when they completely overlap.

3) Was the combined pulse bigger or smaller than the individual pulses when the completely overlapped?

4) Sketch the appearance of the pulses after they have finished overlapping.

When two waves overlap and add together to make a new wave form we say that they interfere. Interference which results in the new wave form being larger than the component waves is called constructive.

5) Do you think the pulses passed through each other or bounced off each other?

With identical wave pulses you really can’t tell, so let us look at non-identical pulses.

Click on the link labeled Animation 3a. Two triangular wave pulses will now interfere. The pulse on the left is shorter and the pulse initially on the right is taller.

6) Sketch the appearance of the pulses just before they overlap.

7) Sketch the appearance of the pulses when they completely overlap.

8) Sketch the appearance of the pulses after they have finished overlapping.

9) Based on this simulation, do the pulses reflect from each other or pass through each other? Explain.

Click on the link Animation 2a. This time two square wave pulses on opposite sides of the string will interfere.

10) Sketch the appearance of the pulses just before they overlap.

11) Sketch the appearance of the pulses when they completely overlap.

12) Was the combined pulse bigger or smaller than the individual pulses when the completely overlapped?

13) Sketch the appearance of the pulses after they have finished overlapping.

Interference which results in the new wave form being smaller than the component waves is called destructive.

Reflection

Start up the simulation titled Wave on a String from the PhET website.

Once the animation has started, move the slider labeled Damping to 0. Click on the radio button labeled Pulse. Click on the green button labeled Pulse to launch a single pulse on the string.

The Clamp on the right is a fixed boundary.

1) What happens to the pulse when it reaches the boundary?

2) Compare the pulse before and after it reflects from the boundary

Click on the Loose End radio button. Now the wave pulse will reflect from a boundary at which the string is allowed to move, which is known as free boundary.

3) How does the reflected wave’s appearance compare to the initial wave? Draw a picture to illustrate your answer

4) Compare the two situations you just observed. What is similar about a wave hitting a fixed boundary and a free boundary? What is different?

Now that we are armed with easy to see pictures in computer simulations, we will explore this phenomena with wave pulses traveling on slinkies where it can be more difficult to observe.

Part 1b: Reflection and Interference of Waves – Slinky

Reflection

Hold the slinky at both ends. Launch a single transverse pulse from one side and carefully observe how it appears after it is reflected.

1) Sketch a diagram showing how the wave pulse looked before and after the reflection.

2) Was the boundary from which the slinky reflected a fixed boundary or a free boundary in this case?

3) Does your observation of the slinky reflecting from this boundary agree with the simulation you ran before?

Now hook the last few coils one end of the slinky over a ring stand so that the slinky is free to slide horizontally on the ring stand. Launch a single transverse pulse and carefully observe how it appears after it is reflected. Note be patient with this step. It can be difficult to see at first.

4) Sketch a diagram showing how the wave pulse looked before and after the reflection.

5) Was the boundary from which the slinky reflected a fixed boundary or a free boundary in this case?

6) Does your observation of the slinky reflecting from this boundary agree with the simulation you ran before?

7) Compare the orientations of the reflected pulses to the incoming pulses for questions 1).

8) Compare the orientations of the reflected pulses to the incoming pulses for questions 4).

There are two distinct ways that a wave can reflect at a boundary. One way is that the reflected wave is on the same side as the incoming wave. In this case, the reflected wave is in phase with the incident wave.

9) Did this occur for question 1) or 4)?

The second way is that the reflected wave is on the opposite side as the incoming wave. In this case, the reflected wave is 180° out of phase with the incident wave.

10) Did this occur for question 1) or 4)?

In question 1) we held the position of the slinky fixed. This is sometimes referred to as a fixed boundary condition.

11) Describe the phase of the reflected wave compared to the incident wave at a fixed boundary condition.

In question 4) we allowed the end of the slinky at the reflected end to move freely. This is sometimes referred to as a free boundary condition.

12) Describe the phase of the reflected wave compared to the incident wave at a free boundary condition.

Interference

Launch single transverse pulses simultaneously and on the same side from each end of the slinky. Observe what happens to the pulses when they meet in the middle. To help you see what's going on, place a small object near the middle of the slinky where the pulses meet.

1) Sketch a diagram showing what happens to the pulses when they meet in the middle.

2) Do the pulses stop when they meet or do they continue moving?

The adding together of the two pulses in the middle is an example of what is called linear superposition. When waves overlap, they simply add together. Linear superposition is also called interference. If the waves reinforce each other as in this case, we call the interference constructive.

3) Were the two pulses in phase or 180° out of phase?

4) Complete the following. Constructive interference occurs when the waves are ________ phase.

Launch single transverse pulses simultaneously and on opposite sides from each end of the slinky. Observe what happens to the pulses when the meet in the middle.

5) Sketch a diagram showing what happens to the pulses when they meet in the middle.

6) Do the pulses stop when they meet or do they continue moving?

If the waves cancel each other as in this case, we call the interference destructive.

7) Were the two pulses in phase or 180° out of phase?