Physlet Illustration: The Doppler Effect for a Moving Source

 

 
A source of waves  is traveling across the screen as shown. The waves travel at speed c. The speed of the source can be controlled with the slider, and is shown as a decimal fraction of c. Is the frequency heard by a stationary observer higher or lower when the source is approaching?  When it is receding?

Hints

  1. Use the slider to set the speed to zero.  Is the same frequency heard by observers on all sides?  
  2. Set the speed to approximately .6 c.  Now does it matter which side of the source the observer is on?
  3. Do more or fewer crests per second pass a stationary observer in front of the source?  Behind the source? 
  4. What happens when the speed of the source approaches the speed of the wave?  Where do shock waves come from?

Reference

See Walker, Section 14-6

Illustration written by Chuck Niederriter