Doppler Effect

How does motion change what we hear?

Source speed
Playback

What do you observe?

  1. Set the source to Stationary. Sketch the wavefront pattern. How does this compare to when the source is moving? What is different about the spacing of the wavefronts at Observer A and Observer B?
  2. Sketch the wavefront pattern you see in front of and behind the moving source. How does the spacing between wavefronts differ on each side?
  3. Observer B is in the path of the approaching source. Describe how the wavefronts reach Observer B compared to Observer A. What does this suggest about the frequency they detect?
  4. Change the source speed to Fast. Describe how the wavefront pattern changes compared to Slow. What does this tell you about how source speed affects the Doppler effect?
  5. Turn on Show Readout. Look at the two oscilloscope screens. Describe the difference in the wave patterns at Observer A and Observer B. What does this suggest about the frequency each observer detects?
  6. The Doppler effect also applies to light. Stars moving away from Earth show a red shift in their spectrum. Using what you have observed, explain why a receding source produces a lower-frequency (longer-wavelength) signal.
Guided Investigation — Doppler Effect

Aim: To investigate whether the motion of a wave source affects the observed frequency and wavelength of the waves it emits.

  1. What variables could you change in this simulation? Which are you keeping constant?
  2. How does the spacing between wavefronts relate to the wavelength and frequency of the wave?
  3. Compare the Stationary wavefront pattern with the Fast moving pattern. What has changed, and what has stayed the same?
  4. Predict: if the source were moving at exactly the same speed as the waves, what would the wavefront pattern look like in front of the source?
  5. How does this simulation help explain why an ambulance siren sounds higher-pitched as it approaches you and lower-pitched as it passes and moves away?