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Topic C.5 · HL only

Doppler Effect: notes and practice questions

2 min read Jump to 1 practice question
Summary
  • This topic extends the understanding of the Doppler effect to include quantitative analysis for sound and mechanical waves.
  • For a moving source, the observed frequency f′f' is given by f′=f(VV±us)f' = f\left(\frac{V}{V \pm u_s}\right), where ff is the source frequency, VV is the wave speed, and usu_s is the source speed.
  • Use the minus sign when the source moves towards the observer, and the plus sign when it moves away.
  • For a moving observer, the observed frequency f′f' is given by f′=f(V±uoV)f' = f\left(\frac{V \pm u_o}{V}\right), where uou_o is the observer speed.
  • Use the plus sign when the observer moves towards the source, and the minus sign when it moves away.

How it is examined

HL Paper 2 uses the mechanical equations directly. May 2025 HL Paper 2 TZ1 question 5(a) gave a stationary loudspeaker and a moving detector: state the direction of motion (1), then calculate the speed (2). The mark scheme is instructive on how strictly the source/observer choice is policed: it awarded zero for using the moving-source equation, awarded 1 mark for an answer of −20 m s⁻¹ with no comment, and 2 marks if the candidate said "20 away from the speaker". It also allowed the SL light approximation as an alternative route because it happens to give the same number here. Never generate a question with both source and observer moving.

Given in the booklet

SL: the light approximation Δf/f=Δλ/λ≈v/c\Delta f/f = \Delta\lambda/\lambda \approx v/c. HL adds both mechanical-wave equations. Choosing the sign in the ±\pm is the student's job and is where the marks go. The May 2023 update changed the wording of these two equations from "velocity" to "speed", so usu_\text{s} and uou_\text{o} are magnitudes.

Key ideas
  • the nature of the Doppler effect for sound waves and electromagnetic waves
  • the representation of the Doppler effect in terms of wavefront diagrams when either the source or the observer is moving
  • the relative change in frequency or wavelength observed for a light wave due to the Doppler effect, where the speed of light is much larger than the relative speed between source and observer, as given by Δff=Δλλ≈vc\dfrac{\Delta f}{f} = \dfrac{\Delta\lambda}{\lambda} \approx \dfrac{v}{c}
  • that shifts in spectral lines provide information about the motion of bodies like stars and galaxies in space

Guiding questions

  • How can the Doppler effect be explained both qualitatively and quantitatively?
  • What are some practical applications of the Doppler effect?
  • Why are there differences when applying the Doppler effect to different types of waves?

Linking questions

  • What are the similarities and difference between light and sound waves?
  • How can the Doppler effect be utilized to measure the rotational speed of extended bodies?
  • What happens if the speed of light is not much larger that the relative speed between the source and the observer?
  • What gives rise to emission spectra and how can they be used to determine astronomical distances?
  • How can the use of Doppler effect for light be used to calculate speed? (NOS)

Practice questions

1 question · 1 medium

Question 1

MediumPaper 2 · calculator6 marks
(a)

An ambulance with its siren sounding passes a stationary observer. The observer hears the frequency of the siren vary from a maximum of 520520 Hz to a minimum of 480480 Hz.

(a) Sketch a graph to show the variation, with time, of the frequency heard by the observer.

[3]
(b)

(b) The speed of sound in air is 340340 m s−1^{-1}. Calculate the speed of the ambulance.

[3]

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Where marks are lost

  • Stopping one step short of the conclusion. Two numbers and no sentence is two marks out of three.
  • Answering a procedure question with a platitude.
  • Losing precision in Paper 1B. Uniquely to this paper, quoting the right number badly loses marks.
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What does Doppler Effect cover in IB Physics?

This topic extends the understanding of the Doppler effect to include quantitative analysis for sound and mechanical waves. For a moving source, the observed frequency f' is given by f' = f((V)/(V ± u_s)), where f is the source frequency, V is the wave speed, and u_s is the source speed. Use the minus sign when the source moves towards the observer, and the plus sign when it moves away.

Is Doppler Effect SL or HL?

Doppler Effect is HL only. SL students are not examined on it.

How do I revise Doppler Effect for IB Physics?

Start from the core idea: this topic extends the understanding of the Doppler effect to include quantitative analysis for sound and mechanical waves. In the exam: hL Paper 2 uses the mechanical equations directly. May 2025 HL Paper 2 TZ1 question 5(a) gave a stationary loudspeaker and a moving detector: state the direction of motion (1), then calculate the speed (2). Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise Doppler Effect?

FourtyFive has 1 Doppler Effect question. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for Doppler Effect practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite Doppler Effect answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

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