What does Doppler Effect cover in IB Physics?

This topic introduces the Doppler effect for sound waves and electromagnetic waves. It describes the change in observed frequency or wavelength due to relative motion between source and observer. Wavefront diagrams can represent the effect when either the source or the observer is moving.

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Topic C.5 · SL and HL

Doppler Effect: notes and practice questions

Q: Is Doppler Effect SL or HL?

Both. SL and HL students study Doppler Effect, and HL goes further: the observed frequency for sound waves and mechanical waves due to the Doppler effect, as given by: moving source f' = f(dfracvv ± u_s), where u_s is the speed of the source moving observer f' = f(dfracv ± u_ov), where u_o is the speed of the observer.

Q: How do I revise Doppler Effect for IB Physics?

Start from the core idea: this topic introduces the Doppler effect for sound waves and electromagnetic 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.

7 min read Jump to 5 practice questions

Summary

This topic introduces the Doppler effect for sound waves and electromagnetic waves.
It describes the change in observed frequency or wavelength due to relative motion between source and observer.
Wavefront diagrams can represent the effect when either the source or the observer is moving.
For light waves, the relative change in frequency or wavelength is given by $\frac{\Delta f}{f} = \frac{\Delta \lambda}{\lambda} = \frac{v}{c}$ .
Shifts in spectral lines provide information about the motion of bodies like stars and galaxies in space.
The Doppler effect has practical applications in medical physics and radar.

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 $\Delta f/f = \Delta\lambda/\lambda \approx v/c$ . 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 $u_\text{s}$ and $u_\text{o}$ 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 $\dfrac{\Delta f}{f} = \dfrac{\Delta\lambda}{\lambda} \approx \dfrac{v}{c}$
that shifts in spectral lines provide information about the motion of bodies like stars and galaxies in space

At HL

the observed frequency for sound waves and mechanical waves due to the Doppler effect, as given by: moving source $f' = f\left(\dfrac{v}{v \pm u_\text{s}}\right)$ , where $u_\text{s}$ is the speed of the source moving observer $f' = f\left(\dfrac{v \pm u_\text{o}}{v}\right)$ , where $u_\text{o}$ is the speed of the observer

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

5 questions · 1 easy · 3 medium · 1 hard

Showing 5 of 5

Question 1

EasyPaper 1A · calculator1 mark

A star in a nearby galaxy is moving towards a stationary observer on Earth with a speed $v$ . The speed $v$ is much less than the speed of light $c$ . The star emits light of frequency $f$ . The frequency measured by the observer is $f + \Delta f$ .

Which graph shows the variation of the frequency shift $\Delta f$ with the speed $v$ ?

Four graphs of Delta f versus v. Graph A is a straight line with a positive gradient passing through the origin. Graph B is a curve, concave down, starting from the origin. Graph C is a curve, concave up, starting from the origin. Graph D is a straight line with a positive gradient and a positive y-intercept.

A. A

B. B

C. C

D. D

Question 2

MediumPaper 1A · calculator1 mark

A police car with its siren on is stationary at a traffic light. A stationary observer is located some distance behind the car. At time $t=0$ , the traffic light turns green and the car begins to accelerate uniformly, moving directly away from the observer, until time $t=T$ . The frequency of the siren is $f_0$ .

The speed of the car is always much less than the speed of sound.

What is the variation with $t$ of the relative change of frequency $\frac{\Delta f}{f_0}$ of the observed sound?

Four graphs of relative frequency change versus time. Graph A shows a straight line starting at the origin and decreasing linearly to a negative value at t=T. Graph B shows a straight line starting at a negative value at t=0 and increasing linearly to zero at t=T. Graph C shows a straight line starting at the origin and increasing linearly to a positive value at t=T. Graph D shows a straight line starting at a positive value at t=0 and decreasing linearly to zero at t=T.

Question 3

HardPaper 2 · calculator18 marks

(a)

An electric ski lift is powered by a motor at the base station. The motor is connected to a 750 V DC power supply by a cable with a total resistance of 0.15 Ω. When operating at full capacity, the motor draws a constant current of 400 A.

(a) Determine the potential difference across the terminals of the motor.

[2]

(b)

(b) The motor has an efficiency of 92%. Calculate the useful mechanical power output of the motor.

[2]

(c)

The ski lift carries skiers up a slope of length 1800 m that rises by a vertical height of 500 m. There are 50 chairs on the ascending side. Each empty chair has a mass of 25 kg and carries, on average, 1.5 skiers of average mass 75 kg. A constant resistive force of 12 kN opposes the motion.

[3]

(d)

(d) Estimate the maximum speed, $v$ , of the ski lift.

[2]

(e)

(e) The lift operates continuously. Estimate the maximum number of skiers that can be transported to the top station in one hour.

[3]

(f)

(f) In an emergency stop, a brake is applied to a large solid steel disc, bringing the lift to a halt from its maximum speed. The total mass of the moving system (chairs, skiers, and cable) is 18 000 kg. Assume all the kinetic energy of the system is converted into thermal energy in the brake disc.

Calculate the temperature rise of the disc.

Data for this question:
Brake disc radius = 0.75 m
Brake disc thickness = 0.10 m
Density of steel = 7850 kg m⁻³
Specific heat capacity of steel = 450 J kg⁻¹ K⁻¹

[4]

(g)

(g) The speed of a chair is monitored using a radar device at the base station that emits microwaves of frequency 30 GHz. It measures the waves reflected from a chair as it moves away. The frequency shift, $Δf$ , for a source moving directly away from a stationary observer can be approximated by the formula $Δf/f ≈ v/c$ . In this radar measurement, this formula gives a good approximation for the shift detected.

Calculate the expected frequency shift.

[2]

Question 4

MediumPaper 1A · calculator1 mark

A star is moving with speed $v$ towards a stationary observer on Earth. The star emits light of frequency $f$ . The frequency measured by the observer is $f'$ . The speed $v$ is much less than the speed of light $c$ .

Which graph shows the variation of the observed frequency $f'$ with the speed $v$ ?

Graph A showing f' on the y-axis and v on the x-axis. The graph is a straight line with a positive gradient passing through the origin.

Graph B showing f' on the y-axis and v on the x-axis. The graph is a straight line with a positive gradient and a positive y-intercept.

Graph C showing f' on the y-axis and v on the x-axis. The graph is a curve with an increasing gradient, starting from a positive y-intercept.

Graph D showing f' on the y-axis and v on the x-axis. The graph is a horizontal line with a positive y-intercept.

Question 5

MediumPaper 1A · calculator1 mark

An astronomer analyses the spectrum of light received from a distant star. Three assertions are made about this spectrum.

I. The dark lines in the star's absorption spectrum correspond to the emission lines of elements in the star's outer atmosphere.

II. The redshift of these spectral lines can be used to determine the star's surface temperature.

III. The existence of discrete spectral lines provides evidence for the quantization of electron energy levels in atoms.

Which of the assertions are correct?

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

Every Doppler Effect question, marked for you

Every answer is marked mark by mark, IB-style, and the AI tutor helps when you are stuck.

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.

Doppler Effect: notes and practice questions

How it is examined

Guiding questions

Linking questions