# The Doppler Effect

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## Objective

Students will quantitatively and qualitatively define the Doppler effect.

#### Big Idea

Students continue to roll through this unit of sound when the see a Doppler Ball and define the Doppler effect in terms of frequency and movement of the observer.

## Context & Equipment Needs

Since this is the second lesson in our sound unit, students have already been introduced to the fundamental characteristics of sound waves. Now that students understand the basics of sound, today's goal is to introduce students to the Doppler effect (HS-PS4-1). Specifically, the lesson starts with a demonstration of the Doppler Effect and then students engage in a paired reading activity. The paired reading activity includes the mathematical definition of the Doppler effect and a few example problems that show students how the equation can be applied (SP5 & SP8). Finally, students get to apply their new knowledge towards the end of class with a hand signals closure activity.

A Doppler Ball is used during the introductory demo.

## Introduction with a Doppler Ball Demo

10 minutes

To give students an opportunity to visualize what today's lesson is all about, I start the lesson off with a demo that shows how pitch changes when a sound source is moving away from or towards you. Before the class enters the room, I have placed the battery into my doppler ball and am ready to roll right into the demonstration.

Once the bell rings, I immediately toss the ball to a student in the back of the room. This way, when the ball soars across the room, students can hear the pitch change from the ball and their attention is immediately focused on the class. I ask the student to throw it back to me and encourage the other students to listen. Then, I toss the ball to the back of the room again, but in the opposite corner. This time around, however, I don't have the student return the ball to me, but I have the student toss the ball to another student of their choice. My goal in this is that students toss the ball not only between the front and back of the room, but also to opposite sides of the room. Giving students this freedom to toss the ball ensures that everyone in the room gets to experience the pitch change from the Doppler Ball.

Once the ball is back in my hands, I ask students to write down their observations of the activity in their notebooks. While I don't collect these observations, I want to give students a few minutes to debrief and critically think about what they just experienced. Their notes don't need to be neat or organized, but they should include a connection to prior material and use of science vocabulary from earlier lessons.

As students are working, I walk around and glance at students' lists. I also ask them questions about their lists. With this student, I might ask her "What do you mean by 'high then low'?" My question is trying to get her to use scientific vocabulary and specifically mention a change in pitch. Or, I might also ask her "What is another way you could describe the loudness of the noise?" This question is meant to prompt her to think about sound intensity.

## Doppler Effect Paired Reading Activity

25 minutes

To define the Doppler effect both quantitatively and qualitatively, students engage in a paired reading activity. I introduce this activity by passing out the paired reading document to each student and explaining that I have already chosen their partners. Partners work best for this activity, and I have a list of who works together based on their current grade in the class. I don't tell the students how I pair them, but I rank the class by overall grade, split the list in half, and then match the first names on each list. To save class time, I print this list and organize the pairs prior to class starting. Because there are an odd number of students, I make an exception and have one group of three. Pairing students forces them to work with someone different and ensures that ability levels are somewhat equal.

Students understand they don't need to scramble for a partner and have the document in front of them. I next share how this paired reading activity works. I expect that the students read one page at a time individually while annotating the text with information they deem important. Students should stop reading when they get to the end of the page. Once both partners have come to the end of page 1, they exchange their annotations and copy down onto their document any ideas that they don't already have. For example, if one student thinks the diagram of a bug swimming in calm water is important and her partner does not identify that, her partner needs to write the diagram down on her own paper. The students then repeat this process until they have completed all five pages of the document. They have approximately 25 minutes to work through this packet, so they should be reading and discussing each page every 5 minutes.

I share with students that this activity has three purposes. The first is to practice reading detailed information in a short amount of time. The AP Physics 1 exam has been redesigned to include more reading, so I want students to be able to practice reading and pulling out important information under a time constraint. This parallels the second goal, which is to build students' stamina for reading physics material. At first glance the reading might seem a bit long, but it is intentionally selected to help students students prepare for the 3 hours they will spend taking the AP exam. The final goal is that students must grasp an understanding of the concepts, vocabulary, and equations used to describe the Doppler effect.

After I'm done giving instructions, I reveal the pairs by simply reading them from the organized list. I have students move so they are sitting with each other, but since they are AP students I let them organize themselves and choose their own seat locations. Once everyone is settled I write the end time of the activity on the front board and begin to circulate the room. My circulation lets me know if students are on task and allows me to redirect students if I hear misconceptions or off-task conversations.

## Hand Signals Closure

5 minutes

To assess how my students are feeling about the Doppler effect, I ask students to show me hand signals. Before I share the prompt, I show students their 3 options: a thumbs-up for agreement, a thumbs-down for disagreement, and a flat hand for uncertainty.

Once the students are ready, I say "I can describe why sounds change pitch when they are coming towards or moving away from me." I then pause and repeat the prompt before asking students to share their hand signals, so that they have a moment to fully embrace and think about how they are feeling. The atmosphere is mostly quiet during this closure so that students are individually assessing their own level of understanding. As students show their hand signals, I make a mental note of how many students are showing a flat hand or thumbs-down. Luckily, everyone felt confident that they could describe the Doppler effect in the context of the world around them.

Knowing that students are confident in their qualitative description, I now say "I can use the Doppler effect equation to solve problems that ask about the heard frequency." This time, there are about six students that show a thumbs-down, so I make a general announcement about doing some supplemental practice problems at home. I offer these problems to all of my students, but really encourage students who are feeling shaky on the concept to take these practice problems home. And, I offer to grade the completed assignment if students want feedback on how well they did on those practice problems.