Properties of Waves
4 marks · EasyA train travels towards an observer while sounding its whistle. The observer notices the pitch of the sound is higher than when the train is stationary.
What is the name given to this effect?
Explain why the observer hears a higher pitch as the train approaches.
State whether the wavelength of the sound increases or decreases for the observer as the train gets closer.
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 Doppler effect · must show uses the term Doppler effect
(b) As the train approaches, the sound waves in front of it are compressed, causing the wavelength to decrease and the frequency to increase. A higher frequency means a higher pitch is heard. · must show mention of compressed waves or decreased wavelength and increased frequency
(c) Decreases · must show wavelength decreases
Common mistakes: Not mentioning compression of wavefronts; Incorrect explanation referencing speed rather than frequency
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States 'Doppler effect'.
B2 (b) · 2 marks · Explains compression of wavefronts, decreased wavelength, increased frequency, leading to higher pitch.
B3 (c) · 1 mark · States that wavelength decreases.
Worked solution
a) The Doppler effect.
b) The train moving towards the observer causes the sound waves ahead of it to be compressed, so the wavelengths shorten and the frequency increases, making the sound's pitch seem higher.
c) The wavelength decreases for the observer as the train gets closer.