The Doppler Effect
3 marks · EasyConsider a moving car sounding its horn.
Describe what happens to the spacing of the sound wavefronts in front of the moving car.
Explain how the change in wavefront spacing affects the pitch (frequency) of the sound heard by an observer in front of the car.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) The wavefronts in front are closer together (compressed). · must show compressed wavefronts
(b) The compressed wavefronts mean a shorter wavelength, which leads to a higher frequency. The observer hears a higher pitched sound. · must show shorter wavelength, higher frequency or pitch
Common mistakes: Describing only louder, not frequency/pitch change.; Not relating spacing to frequency.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes the wavefronts as closer together (compressed) in front of the car.
B2 (b) · 1 mark · Mentions the wavelength is shorter.
B3 (b) · 1 mark · Correctly links shorter wavelength to increased frequency or pitch for the observer.
Worked solution
a) The sound wavefronts in front of the car are closer together (compressed). b) Because the wavefronts are closer, their wavelength is shorter. Shorter wavelength means higher frequency, so the observer in front hears a sound with a higher pitch.