You’ve heard it a hundred times: an ambulance races past and its siren seems to drop in pitch — neeeee-owwww. The siren itself never changes. What changes is the relative motion between you and it. That change in the frequency you observe is the Doppler effect.
📘 What you need to know
The Doppler effect (or Doppler shift) is the change in observed frequency due to the relative motion between a source of sound or light and an observer
The source keeps emitting at a constant frequency — only the frequency the observer receives changes
Source moving towards the observer → wavelength shortened → observed frequency higher (higher pitch)
Source moving away from the observer → wavelength stretched → observed frequency lower (lower pitch)
The wave speed does not change — with v = fλ fixed, a shorter λ must mean a higher f
It happens to all waves, including light: moving away = red-shifted, moving towards = blue-shifted
What Is the Doppler Effect?
Start with the boring case, because it sets the baseline. If a sound source (say, an ambulance siren) and an observer are both stationary, the wavefronts spread out as evenly spaced circles. The observer receives waves with exactly the same frequency and wavelength as the source emits — nothing interesting happens.
Stationary source, stationary observer: the wavefronts are evenly spaced circles, so the observer receives exactly the wavelength and frequency the source emits.
Now set the source or the observer moving relative to each other, and the observer suddenly measures a different frequency from the one the source emits. That’s the Doppler effect:
The Doppler effect is the change in observed frequency (and wavelength) caused by the relative motion between a wave source and an observer.
Notice the careful wording — the frequency appears to change. The siren, whistle or lamp emits at a constant frequency throughout; it’s what arrives at the observer that shifts.
A Moving Source: Squashed and Stretched Wavefronts
Here’s the mental movie. A moving source emits a wavefront, then chases after it before emitting the next one. Each new circle is centred a little further along, so:
In front of the source, the wavefronts get squashed together — the wavelength decreases to λ − Δλ, and the observed frequency increases
Behind the source, the wavefronts are stretched apart — the wavelength increases to λ + Δλ, and the observed frequency decreases
A source moving at speed vs: each wavefront is centred where the source was when it emitted it, so the circles bunch up ahead (λ − Δλ) and spread out behind (λ + Δλ). The observer in front measures a higher frequency; an observer at P measures a lower one.
Here Δλ is the change in wavelength — and the bigger the change, the bigger the Doppler shift. Why must the frequency change too? Because the wave speed stays the same (it’s set by the medium, not the source). The wave equation v = fλ then leaves no choice: if λ shrinks and v is fixed, f must rise, and vice versa.
It Happens to Light Too
The Doppler shift is observed by all waves — sound and light. For electromagnetic waves, the vocabulary changes but the physics is identical:
A light source moving away is red-shifted — its wavelengths shift towards the red end of the electromagnetic spectrum, the equivalent of sound dropping to a lower frequency
A light source moving towards you is blue-shifted — wavelengths shift towards the blue end, the equivalent of a higher-frequency sound
The names make sense once you remember that red light has a longer wavelength than blue light — so “shifted red” means “stretched”, and “shifted blue” means “squashed”.
source moves away
wavelength stretched
sound: lower pitch
for light
RED-SHIFT
source moves towards
wavelength squashed
sound: higher pitch
for light
BLUE-SHIFT
🎨 Drawing a Doppler wavefront diagram
Stationary source: concentric circles, all sharing one centre, evenly spaced
Moving source: each circle’s centre sits where the source was when it emitted that wavefront — so the centres form a trail behind the source’s current position
Check the squash: the circles must bunch up on the side the source is moving towards, and spread out behind
Label it: mark λ − Δλ in front and λ + Δλ behind, with the velocity arrow on the source
Quick recap: relative motion changes the frequency an observer receives — squashed wavefronts (towards) mean higher f, stretched ones (away) mean lower f — while the source’s emitted frequency and the wave speed never change.
WE 1
An ambulance drives past a stationary pedestrian with its siren on. Describe and explain what the pedestrian hears as the ambulance approaches and then moves away.
Approaching
The wavefronts ahead of the ambulance are squashed together, so the wavelength reaching the pedestrian is shorter. The wave speed is unchanged, so by v = fλ the observed frequency is higher — the siren sounds higher in pitch
Moving away
Behind the ambulance the wavefronts are stretched apart — longer wavelength, so lower observed frequency and a lower pitch
pitch drops as the ambulance passesAll the while, the siren itself emits at a constant frequency — only what the pedestrian receives changes.
WE 2
A train sounds its horn while moving away from a person standing on a platform. State what happens to each of the following, as measured by the person: (a) the wavelength of the sound, (b) the frequency of the sound, (c) the pitch heard, and (d) the frequency emitted by the horn itself.
Part (a)
The source is moving away, so the wavefronts behind it are stretched
wavelength: longerPart (b)
Wave speed unchanged, so from v = fλ a longer wavelength means
frequency: lowerPart (c)
Lower frequency is heard as
pitch: lowerPart (d)
The horn keeps emitting exactly as before
emitted frequency: unchanged
💡 Top tips
The wave speed never changes in Doppler questions — the wave equation v = fλ (in your data booklet) is what links the λ change to the f change
Two frequencies live in every Doppler question: the emitted one (constant) and the observed one (shifted) — always say which one you mean
The squash is always on the side the source is moving towards — sketch the trailing circle centres and you can’t get it wrong
Red-shift = away, blue-shift = towards — anchored by “red light has the longer wavelength”, so stretched waves look redder
⚠ Common mistakes
Saying the source’s frequency changes — it doesn’t; only the observed frequency shifts
Claiming the sound “travels faster” towards an approaching observer — the wave speed is fixed by the medium
Squashing the wavefronts on the wrong side — they bunch up in the direction of motion
Swapping red-shift and blue-shift — remember stretched = longer λ = red end of the spectrum
Up next: putting numbers on it — the Doppler effect of light and the equation Δf/f = Δλ/λ ≈ v/c that lets astronomers clock a star’s speed from its spectrum.
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