Doppler shift of sound
- The whistle of a train or the siren of an ambulance appears to decrease in frequency (sounds lower in pitch) as it moves further away from you
- This frequency change due to the relative motion between a source of sound or light and an observer is known as the doppler effect (or doppler shift)
- When the observer (e.g. yourself) and the source of sound (e.g. ambulance siren) are both stationary, the waves are at the same frequency for both the observer and the source
Waves emitted from a stationary source
According to the observer stationary relative to the source, the frequency is equal to that measured by the source
- When the source starts to move towards the observer, the wavelength of the waves is shortened
- The sound therefore appears at a higher frequency to the observer
Waves emitted by a moving source
The source and observer disagree on their measurements of the frequency of the wave
- Notice how the waves are closer together between the source and the observer compared to point P and the source
- If the observer was at point P instead, they would hear the sound at a lower frequency due to the wavelength of the waves broadening
- The frequency is increased when the source is moving towards the observer
- The frequency is decreased when the source is moving away from the observer
Worked example
A cyclist rides past a stationary observer ringing their bell as they ride away.
Which of the following accurately describes the Doppler shift caused by the sound of the bell as the cyclist moves away from the observer?
Wavelength | Frequency | Pitch | |
A | Shorter | Higher | Higher |
B | Longer | Lower | Higher |
C | Shorter | Higher | Lower |
D | Longer | Lower | Lower |
Answer: D
- As the cyclist rides away from the observer, the wavelength increases or becomes longer
- This rules out options A and C
- A longer wavelength means a lower frequency (from the wave equation )
- A lower frequency corresponds to a sound of lower pitch
- Therefore, the correct answer is D