Doppler effect — Physical Sciences | Learn on Stanmorephysics

4 exam-style question sets on Doppler effect (Paper 1), each with a hint and a fully worked answer. The app holds 20 questions on this section in total, including variants of every set below, and lets you mark yourself part by part.

Moving source — wavelength graphDoppler shift & spectraMoving source — graph of detected frequency vs speedRed & blue shift

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Question 1

Moving source — wavelength graphApplication12 marksPaper 1

An ambulance drives along a straight road at constant velocity with its siren on. A learner stands still on the pavement and uses a sound sensor to record the wavelength of the siren's sound.

The wavelength-time graph (NOT drawn to scale) shows $λ_{L}$ , the wavelength recorded by the learner, and $λ_{S}$ , the wavelength of the sound emitted by the siren: $λ_{L}$ = 0,75 m and $λ_{S}$ = 0,80 m.

Take the speed of sound in air as 340 m·s⁻¹.

aWrite down the NAME of the phenomenon illustrated by the difference between the two wavelengths. (1)
bIs the ambulance moving TOWARDS the learner or AWAY FROM the learner? Use the graph to give a reason for the answer. (2)
cCalculate the frequency of the sound emitted by the siren. (3)
dCalculate the magnitude of the velocity of the ambulance. (6)

This question has a diagram, shown in the app.

Hint

Get both frequencies from v = fλ, then compare them with the Doppler equation for a moving source.

Worked answer

a) The Doppler effect. ✓

b) TOWARDS the learner. ✓ The recorded wavelength (0,75 m) is SHORTER than the emitted wavelength (0,80 m) — wavefronts ahead of a source moving towards an observer are compressed. ✓

c) $v = f λ$ ✓
$f_{s} = \frac{340}{0 , 80}$ ✓ $= 425$ Hz ✓

d) Frequency recorded by the learner:
$f_{L} = \frac{v}{λ _{L}} = \frac{340}{0 , 75} = 453, 33$ Hz ✓✓
Doppler equation (source moving towards stationary listener):
$f_{L} = \frac{v}{v - v _{s}} f_{s}$ ✓
$453, 33 = \frac{340}{340 - v _{s}} (425)$ ✓
$340 - v_{s} = 318, 75$ ✓
$v_{s} = 21, 25$ m·s⁻¹ ✓

Modelled on Nov 2024 Paper 1, Question 6 — same skills, new scenario.

Question 2

Doppler shift & spectraComprehension2 marksPaper 1

Light from a distant galaxy is analysed on Earth. The dark lines in its spectrum appear at LONGER wavelengths than the same lines produced by a source in the laboratory.

Which combination is CORRECT for the motion of the galaxy and the frequency of its light as measured on Earth?

Hint

A longer observed wavelength means the source recedes; frequency and wavelength change in opposite ways.

Worked answer

D. A shift of spectral lines to longer wavelengths (a red shift) occurs when the source moves away from the observer. Since $c = f λ$ and $c$ is constant, a longer wavelength means a lower observed frequency. A and B require motion towards Earth, which would shorten the wavelength. C pairs recession with a higher frequency, contradicting $c = f λ$ .

Modelled on Nov 2024 Paper 1, Question 1.6 — same skill, new scenario.

Question 3

Moving source — graph of detected frequency vs speedApplication12 marksPaper 1

A physics club investigates how the speed of a moving sound source affects the frequency of the sound that a stationary listener detects.

A miniature train at a theme park moves along a straight track at a constant speed while its whistle, which emits sound of a CONSTANT frequency, is sounded. A microphone connected to a frequency meter is mounted next to the track. The runs are repeated at several different constant speeds, first with the train moving TOWARDS the microphone (curve P) and then with the train moving AWAY from the microphone (curve Q).

At a speed of 20 m·s⁻¹ the meter records 765 Hz for curve P and 680 Hz for curve Q. Both curves meet the vertical axis at 720 Hz.

aState the Doppler effect in words. (2)
bWrite down the independent variable for this investigation. (1)
cWrite down ONE variable that must be kept constant during this investigation. (1)
dIdentify the curve, P or Q, that was obtained while the train moved TOWARDS the microphone. Give a reason for the answer. (2)
eUse the two readings taken at 20 m·s⁻¹ to calculate the speed of sound in air on the day of the investigation. (6)

This question has a diagram, shown in the app.

Hint

Write the Doppler equation once for approach and once for recession at the same source speed, then divide the two equations to eliminate the source frequency.

Worked answer

a) The Doppler effect is the change in frequency (or pitch) of the sound detected by a listener because the sound source and the listener have different velocities relative to the medium of sound propagation. ✓✓

b) The speed (velocity) of the train (the sound source). ✓

c) Any ONE: the frequency of the whistle / the speed of sound in air (same weather conditions) / the same microphone position. ✓

d) Curve P. ✓ A source moving towards the listener produces a detected frequency HIGHER than the source frequency, and P lies above 720 Hz. ✓

e) Towards: $f_{P} = \frac{v}{v - v _{s}} f_{s}$ ✓ Away: $f_{Q} = \frac{v}{v + v _{s}} f_{s}$ ✓
Dividing: $\frac{765}{680} = \frac{v + 20}{v - 20}$ ✓
$765 (v - 20) = 680 (v + 20)$ ✓
$85 v = 28900$ ✓
$v = 340$ m·s⁻¹ ✓ (formula ✓ substitution ✓ answer+unit ✓)

Modelled on Nov 2025 Paper 1, Question 6 — same skills, new scenario.

Question 4

Red & blue shiftComprehension2 marksPaper 1

Spectral lines in the light received from galaxy G are observed at SHORTER wavelengths than the same lines produced by a source in a laboratory.

Which ONE of the following conclusions is CORRECT?

Hint

Shorter observed wavelength = blue shift — what relative motion causes that?

Worked answer

C. A shift to shorter wavelengths is a blue shift, which the Doppler effect produces when the source approaches the observer. A describes a red shift (longer wavelengths). B is backwards — shorter wavelength means higher observed frequency, since $c = f λ$ with $c$ fixed. D is impossible: the speed of light in a vacuum stays 3,0 × 10⁸ m·s⁻¹ regardless of the source's motion. Modelled on Nov 2025 Paper 1, Question 1.6 — same skills, new scenario.

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Grade 12 Physical Sciences: Doppler effect

Question 1

Question 2

Question 3

Question 4

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