The Doppler Effect
2 marks · NormalDescribe what happens to the wavelength and frequency of sound waves ahead of a car as it moves towards a stationary observer.
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The wavelength decreases and the frequency increases ahead of the moving car.
Also accept: Wavelength gets shorter; frequency gets higher.
Common mistakes: Mixing up frequency and wavelength changes.; Describing changes for behind rather than ahead of the car.
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Marking points
B1 · 1 mark · States wavelength decreases (is compressed) ahead of the car.
B2 · 1 mark · States frequency increases ahead of the car.
Worked solution
As the car moves towards the observer, the wavefronts are compressed ahead of the car. This causes the wavelength to decrease and the frequency to increase.