The Doppler Effect
3 marks · NormalA football referee blows the whistle while jogging towards some players. The players report hearing a whistle that is slightly higher in pitch than normal.
Explain, using the idea of wavefronts, why the frequency of sound is heard as higher.
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As the referee moves towards the players, each new wavefront of sound is emitted from a position closer to the players, causing successive wavefronts to be closer together (compressed). This means the wavelength is shorter and the frequency is higher, so players hear a higher pitch.
Also accept: Wavefronts are emitted closer together, so wavelength is less and frequency greater, increasing pitch for players.
Common mistakes: Describing change in amplitude rather than frequency.; Not mentioning wavefront positions or compression.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Mentions wavefronts are closer together/compressed as the source moves towards the players.
M2 · 1 mark · Clearly connects that compression of wavefronts means wavelength is shorter.
A1 · 1 mark · Links shorter wavelength to higher frequency and increased pitch.
Worked solution
As the referee moves towards the players, each new wavefront is emitted from a location closer to the players than the previous wavefront, which means wavefronts reach the players more frequently—the wavelength is shorter. Since frequency is inversely related to wavelength (for a given speed), the frequency increases, so the pitch is higher.