The Doppler Effect

Grade Level

6-12

Clean-Up

5 minutes

Learning Goals

Students will be able to:

• Draw pictures of high and low frequency wave fronts
• Explain why the pitch of a car horn changes as it approaches and then drives past
• Explain the Doppler effect, with diagrams, and give examples of where it can be heard

Materials not in Kit

Computers*

*If there aren’t enough computers for every student it’s possible for the sound simulation activity to be done as homework or as a teacher demonstration.

Set-Up

Gather materials and set up computers with the PhET simulator “Sound” and YouTube videos:

→ Doppler Effect – car engine https://youtu.be/8RS2ToLPDZ8?t=34 (33 seconds in)
→ Doppler Effect – car horn http://www.youtube.com/watch?v=a3RfULw7aAY

Introduce the Activity

Students will investigate the “Sound” simulation and explore how the waves change as the parameters are adjusted, then they will answer questions 1 and 2 of their worksheet.

Doing the Activity

Video Demonstrations
1. Show the video of the car engine, starting at the 33 second mark. Ask the students what they observe about the sound.

Doppler Effect – As the source of a wave (sound or light) approaches an observer, the observer sees/hears a higher frequency than the source actually is emitting.   As the source moves away from an observer, the observer sees/hears a lower frequency wave than the source actually is emitting.

2. Show the excerpt several times until the class comes to the idea that pitch is higher as the car heads towards you, and lower as it drives away. Students may mention that the car is loudest when it’s right in front of the man with the microphone, but the Doppler effect deals with pitch change, not volume.

3. Show the car horn video and discuss what is heard. This doesn’t need to be shown as many times as the first video because most of the observing was done the first time around.

Tuning Fork Demonstration

If you don’t have a tuning fork on a string, video of this demo is available for download here: http://physics.wfu.edu/demolabs/demos/3/3b/3B4018.html

1. Hold the string of the 883 Hz tuning fork and swing it around your head. It should vibrate loud enough for the class to hear.
2. Students will answer questions 3 and 4 of their worksheet.

Note: Question 3 asked them to describe what was heard – this was already answered as class discussion, but is here to reinforce the idea for every student.

Drawing Wave Fronts

1. Draw wave fronts for a high frequency wave and for a low frequency wave on the board. (The high frequency wave fronts should have waves closer together than the low frequency wave front.)
2. Ask students to think about the car moving and try to come up with a theory as to why the pitch sounds higher as the car approaches and lower as it drives away.
3. Have the class report their theories.

Discussion

Discuss the previous demonstrations and how they are called the “Doppler Effect.” Explain the concepts to the students.

Students will answer question 5 on the worksheet.

Explanation

When drawing a wave front one can see that if the source is moving towards the observer, the source is closer each time it emits a new wave front. That brings the wave front closer to the previous wave front than it would have been if the source were stationary. So you can see that the wave fronts are closer together as they reach the listener so they are arriving at a higher frequency.

Key Lesson Terminology

Pitch – How low or high a tone sounds to a person
Frequency – wiggles per second (moves back and forth)