Wave Reflection & Interference

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Students will be able to explain wave behavior after reflections and due to different forms of interference.

Big Idea

Students map their understanding of wave interactions and then collaboratively solve wave interference problems.


As we move through this unit on Simple Harmonic Motion, today's goal is to define the behavior of a wave when it encounters interference (HS-PS4-1). The lesson starts with students sharing what they observed in a simulation during the previous lesson. They create concept maps before I share some common rules about wave interference. Finally, the lesson ends when students work together to collaboratively solve problems and apply their new wave interference knowledge (SP5 & SP6).

Concept Mapping: Waves

10 minutes

As students enter the room I greet each student and ask him or her to take a sheet of blank, white, 8.5" x 11" paper from a pile that I'm holding. If I'm holding the paper and standing near the door when they walk in, I get the opportunity to connect with each student at the start of class. Students may choose to do their concept maps digitally if there are computers, tablets, or smartphones available.

Once the start bell has rung, I tell students that they are using their sheet of paper to do a concept map on wave reflection and interference. Although we've done concept maps in this class before, I briefly explain that concept maps can be used as a tool to assess prior knowledge and show a blank model on the front board.

I make sure each student grasps the concept map idea by checking for understanding. I ask "Does everyone get this?" and then pause for about three seconds. If someone has questions I address them, but otherwise we move on. This means it's time for students to work and create their own maps! I keep the learning environment quiet for this activity, otherwise I've found that the students are less authentic about sharing their own thoughts. Students get about five minutes to work on their individual maps before I ask that the maps get put aside for later in the class hour (read the next section to find out what we do with them).

This student sample of a concept map shows that students not only understood the activity, but also have a solid understanding of some of the material that we've covered about waves. While I would have preferred this student put more emphasis on any patterns that they observed during the previous lesson, I'm impressed by their distinction between transverse and longitudinal waves.

Wave Interference & Reflection Direct Instruction

20 minutes

It's time for students to take out their notebooks to copy down the key concepts of wave interference and reflection. My students are operating under the expectation that they must write down key points from the presentation. This expectation of how to take notes has been outlined and ingrained in their learning since freshman year. Because these are AP students, how they organize their notes (notebook, binder, etc.) is a decision the individual student gets to make. I assume at this point in their high school careers they have an established system to stay organized.  

I display the reflection & interference notes to help the students understand what they need to write down. Not only do my notes serve as a guide for students, but they also include pictures that help students visualize constructive interference, destructive interference, and reflections. I have a hard copy of the slides in my hands as I circulate throughout the room that includes notes and questions I want to ask the students to stimulate critical thinking (viewable when the PowerPoint is downloaded). These notes help me to stay focused and ensure I mention the highlights as we progress through each slide.

I usually have a lot of interaction with my students throughout the presentation, and it's purposely short in content because my goal is to support what students saw in yesterday's simulation. The notes are meant to give students a sense of how waves can interact with each other as the occupy the same space at the same time. After I explain this concept to students and we define the principle of superposition, students define constructive and destructive interference. With these definitions, students then explore the interference that occurs when waves come in contact with free and fixed boundaries.

After the notes are complete, I ask students to take out their concept maps from the start of class, and revise them. I share with students the idea that since we've now reviewed the behavior of waves, it is time to amend, add, and complete their concept maps. They must make any changes and additions to their maps in a different color so that I can see their path in learning. I once again have students work individually, since I collect these maps at the end of class. Collecting these maps allows me to ensure they included the concepts of constructive interference, destructive interference, fixed boundaries, and free boundaries (as this student did).

Wave Interference Collaborative Problem Solving

20 minutes

As closure and an informal assessment, students have the rest of the class to start tonight's homework (also available as a Word document). I call it informal because I don't want students to get nervous that it will count as a quiz or test grade. Since my goal is to assess their level of knowledge about waves, specifically wave reflection and interference, I collect and grade the assignment for accuracy at the start of the next class. Not only do I want to give students personalized feedback on this homework assignment, I also want to check the pacing of the course and make sure my students are ready to move on to the next lesson.

Students remain in their seats as I pass out a copy of the homework to each student. This is an assignment that needs to be completed by each individual, although they may use those people seated around them as a resource while working in class. I encourage collaboration throughout their work time and since students are quite comfortable with each other at this point in the year, they all work together well. Also, I walk around to offer help or problem solve with the students as they are working. You might hear me say "Well, what does your neighbor say about the definition of a longitudinal wave?" Or, I might say "Constructive interference doesn't apply to crests only - read the definition of constructive interference again." My style is to lead the students to the answer, not just provide it for them. That being said, if a student is continually struggling and in obvious need of being shown the answer, I accommodate him or her.  

This is our closure activity for today and it's meant to have students apply their newly learned knowledge from the paired reading activity. Most students are able to complete the first 20 to 25 questions, depending on the individual. I am also trying to take a step towards a flipped classroom. I like students to have me as a resource when they work through problems, and I think it helps them build confidence. I find that a combination of work time (that lasts right up until the bell rings, since there wasn't a single student that was able to complete the assignment) and in-class learning best fits the needs of my students.