The Doppler effect
3 marks · EasyA van is moving while playing music from a speaker attached to its roof.
Describe what would happen to the frequency of the music heard by someone standing in front of the moving van, compared to someone standing behind it.
Explain why this happens.
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Show answer
(a) The person in front hears a higher frequency (higher pitched music), and the person behind hears a lower frequency (lower pitched music).
(b) Waves are compressed in front (shorter wavelength), stretched behind (longer wavelength), changing frequency.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States higher frequency/pitch in front.
B1 (a) · 1 mark · States lower frequency/pitch behind.
B1 (b) · 1 mark · Explains in terms of wave compression/stretching or wavelength change.
Worked solution
a) The person in front hears a higher pitch, and the person behind hears a lower pitch. b) This is because the moving van compresses the waves in front and stretches them behind, changing their wavelength and frequency.