In the previous lesson, Wave Hello, students determined that the speed of a wave depends on only the medium. Now students learn about constructive interference and destructive interference as they explore what happens when two slinky waves meet at the same place at the same time. To do this activity, students work in teams of three (two to opperate the slinky and a third to video record the interaction). They need a double-length slinky and an Apple iPhone with slom-motion capabilities (at least 50% of my students have an Apple iPhone).
As students experiment with wave behavior and interactions, this will set the stage for further exploration of NGSS HS-PS4-5: Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy.
To come to understand wave interference with a double-length slinky, CCSS Math Practice 2: Reason abstractly and quantitatively and Math Practice 7: Look for and make use of structure are involved as well as NGSS Science Practice 2: Developing and using models, Science Practice 4: Analyzing and interpreting data and Science Practice 7: Engaging in argument from evidence as students have to apply what they see of the slinky's behavior to generalize and predict other wave behavior.
Class starts with a review of the homework assigned last lesson, Wave Hello. I project the solutions on the board with Wave Homework Solutions under my document camera. While students check their answers I walk through the room with grade book in hand and check to see that students completed the assignment. For now, I assign a grade based on effort. Tonight, students get a homework assignment which I assess for understanding of wave vocabulary.
I then ask for two volunteers to operate a slinky for a class demo and instruct the rest of the class to position themselves so they can see the slinky. We spend a few minutes in review of basic wave vocabulary. I ask the students with the slinky to produce a transverse wave. I point out features such as the amplitude and the velocity of the wave. I also remind students that the medium of this slinky wave is the slinky itself.
Then I tell the students that today's goal is to understand what happens when two waves meet at the same place at the same time. I also have the objective written on the board and I have students write it in their notebooks. Here is the amazing thing about waves, I tell the students; No matter they type of wave, sound, light, earth quake, slinky, if we can understand the behavior of one type of wave, that behavior applies to ALL waves. A wave is a wave is a wave. So what they learn today about waves on a slinky applies to our understanding of how sound and light waves interact. This leads to an understanding of how various technologies work, including radars, microwaves and mobile phones; it even explains how rainbows form.
It is essential that I teach the students how to make a good wave pulse on the slinky. Without a good, well defined pulse, it is difficult to see the waves interact. I instruct the students to use the tiles on the floor as a guide and a measuring devise. I have students pick a line on the floor as their starting position. This is the equilibrium position. To make a good pulse, I have students start at that equilibrium position and then move their hand sideways across the floor two tiles and back to the equilibrium position as fast as they can. This student video shows good form.
Once students understand how to make a good wave pulse, we move onto the rules for slinky use and the wave activity.
In the past, I had serious issues with slinkys becoming tangled and kinked. Every year I would loose 4 or 5 of them! Usually this was due to students waving the slinkys wildly through the air as they tested the limits of their wave making abilities. I found that if the slinky could be kept on the floor, the incidence of kinking reduced dramatically. So, a few years ago, I introduced my Slinky Rules. Here is how it works: I hold up the rules and contract sheet (every group gets a copy) and I tell my students that every group member needs to memorize the rules and sign the contract. They do not get a slinky until this is done. I tell them with a wink and a nod, "This is a legally binding document, so make sure you understand what you are signing".
Next, I break students into teams of three (two to operate the slinky and a third to video record the interaction). They need an Apple iPhone with slow-motion capability (at least 50% of my students have one). First I have everyone with an Apple iPhone raise his or her hand. I count off students with a raised hand till I reach a number that is one-third of the class (e.g. if I have 24 students, I count out 8). Those students stand up and I hand them a Slinky Rules and Contract sheet. The rest of the class then joins one of the standing students so that students are in groups of three.
It usually takes few tries before the groups get their slinky, as can be seen in Slinky rules 1 and Slinky rules 2. Inevitably, one of the members does not take the rules seriously. But once they understand the gravity of the situation and how important it is to keep their slinky un-kinked, they can proceed with the wave interference activity. When students get the rules right, I hand them their slinky and the Wave Behavior Activity sheet. This seemingly silly exercise has helped with maintaining my slinkys, as students get the idea that proper handling of the slinky is serious business and that any tangles or kinks must be fixed by them.
I remind students about the goal of the activity: to see what happens when two waves meet at the same place at the same time. I ask students to use their iPhones to capture that moment.
Because my classroom cannot accommodate 5-10 giant slinkies stretched out, students need to work in the hallway. They spread out and I watch them work. Of course students want to play around with the slinky and see all the different things they can do with it. I let them play for a few minutes as it is fun. However, after a few minutes, I nudge students to get on task.
Overall, it takes about 25 minutes for the students to work through the Wave Behavior Activity. With two students holding either end of the slinky, they both make a transverse wave pulse with equal amplitudes on the same side.
The third student video records the waves interact. They watch the video in slow-motion and draw a picture of what they see on the worksheet. They repeat this for different sized waves.
Then they repeat this, but this time they put the wave pulses on opposite sides of the slinky. Again, they draw what they see on the space provided on the worksheet.
After 25 minutes, the groups finish the worksheet. They return their unkinked slinky to me and sit so that we can debrief the activity as a class.
After the students have finished the activity and returned the slinky, I have them return to there seats so we can review what they should have seen. During the activity, I recorded one group making their waves. I replay those videos for the class to see, pausing at the moment where the waves meet. I encourage students to fix any mistakes they have on their papers using a different color pen.
First I show the Student video - same side, which is in real-time and then repeated but in slow motion. We count the tiles to see that the two wave amplitudes, which are 1.5 tiles each, combine to give an amplitude of 3 tiles.
Next, I show the Student video - opposite side and we see that the two waves almost perfectly cancel each other out. They have slightly different wavelengths so the slinky is not perfectly flat at the moment of interference.
Finally, we look at the Student video - reflection to see that the pulse flips to the other side after bouncing of the student's hand. This is important for later understanding standing waves.
To finish our time together, I have students take an exit quiz where they have to predict how two waves interact. I use the PhET simulation, "Wave on a string" (PhET provides a series of high quality physical science simulators and is provided by The University of Colorado).
I set this simulation as shown below (Pulse, Damping: none, Tension: low, Rules checked) and create two wave pulses. I measure the amplitudes of the waves and I have students draw the pulses on paper and then predict what the interference of the two waves should look like, along with what it's amplitude should be.
I then play the simulation so they see if their answers are correct. Depending on time, I play a variety of wave pairs with various amplitudes. Students are to draw and make predictions on each of them. I collect these sheets to correct and assess understanding.