Module 8—Mechanical Waves

Lesson 6—The Doppler Effect

 

Get Focused

 

A photograph shows a jet plane coming out of a cloud that it has created by flying at the speed of sound.

US Navy photo by Ensign John Gay

The plane in this photo is travelling near the speed of sound. Just as a boat moving through water produces a bow wave, the plane will produce a bow wave as it moves through the air. Normally, the bow waves propagate outward in a V-shape behind the object that is producing them. However, in this case, the plane is travelling as fast as the bow waves (the speed of sound). When this occurs, the bow waves start to “pile up” or store energy, creating what is known as a sonic boom.

 

In a sonic boom, the leading pressure waves produced by the plane are compressed, forming a shock wave that is seen in the photograph as compressed water vapour in the form of a cloud. The wavelength of the bow waves change, depending on how fast the object producing them is moving. You may have observed this with your ears when a fast-moving object, such as a plane, train, or race car, passes by. The sound it makes as it approaches you is different in frequency and wavelength than the sound it makes once it has passed by. This is called the Doppler effect, and it is also responsible for producing a sonic boom. Why does the frequency of the sound waves change when the source producing them moves? What is the relationship between the speed of sound, its frequency, and its wavelength? How are these relationships described mathematically by the Doppler equation?

 

As you work through this lesson, keep the following questions in mind:

Module 8: Lesson 6 Assignments

 

Your teacher-marked Module 8: Lesson 6 Assignment requires you to submit a response to the following:

You must decide what to do with the questions that are not marked by the teacher.

 

Remember that these questions provide you with the practice and feedback that you need to successfully complete this course. You should respond to all the questions and place those answers in your course folder.