Unit 8 · Lesson 8.3.1
The Doppler Effect
Describe the Doppler effect for a moving wave source and predict how the observed wavelength and frequency change as the source approaches or recedes.
What the Doppler Effect Is
Doppler effect
- The change in the observed wavelength and frequency of a wave when the source of the wave moves relative to the observer.
Wave types
- The effect applies to sound, light and microwaves, for any type of wave from a moving source.
Police siren
- The pitch of a siren changes as a moving car passes, because the observed wave pattern changes.
Red-shift
- Light from distant galaxies is observed at longer wavelengths because those galaxies are moving away from us.
Source Moving Towards or Away
1
When a source moves towards an observer, the waves bunch together, so the observed wavelength decreases and the frequency increases.
2
When a source moves away from an observer, the waves stretch out, so the observed wavelength increases and the frequency decreases.
3
The speed of the emitted waves does not change, so a change in observed wavelength always means a change in observed frequency.
4
An observer behind a moving source therefore sees stretched waves with a lower observed frequency.
Positions of the Observer
Positions of the Observer
In front
- A stationary observer in front of a moving source detects a higher frequency than the emitted frequency.
Behind
- A stationary observer behind a moving source detects a lower frequency than the emitted frequency.
Wave speed
- The speed of the waves through the medium stays the same, so only the observed wavelength and frequency change.
Stationary source
- A stationary source emits waves symmetrically in all directions, all with the same wavelength.
Exam Checklist & Tips
8.3.1 – Key Points for the Exam
- Doppler effect: An apparent change in wave frequency or wavelength caused by relative motion between source and observer.
- Observed frequency: The frequency of a wave as it is detected by an observer.
- Red-shift: An increase in the observed wavelength of light, moving it towards the red end of the spectrum.
- Blue-shift: A decrease in the observed wavelength of light, moving it towards the blue end of the spectrum.
- Wavelength: The distance between two consecutive points in phase on a wave.
Common Mistakes
- Saying that the waves themselves change speed; the apparent change comes from the motion of the source.
- Mixing up approaching and receding sources: an approaching source has a higher observed frequency.
- Stating that the Doppler effect applies only to sound; it applies equally to light and microwaves.