C.5 Doppler effect Topic Practice

IB Physics syllabusPhysics SL/HLFirst assessment 2025

Practise Doppler effect questions by reading wavefront or spectrum evidence and calculating speed from frequency or wavelength shifts.

Exam points

use `Delta lambda / lambda = v / c` or the sound Doppler equation to calculate speed with correct units
use a wavefront or spectrum diagram to decide whether the source moves toward or away from the observer
use redshift data from a spectrum or data table to calculate recessional speed or estimate Hubble constant

[Maximum number: 1]

A source moving with speed v away from a stationary observer emits light of wavelength $\lambda$ . The wavelength received by the observer is $\lambda+\Delta \lambda$ . The speed v is much less than the speed of light.

Which graph gives the variation of $\Delta \lambda$ with v ?

(a)

Explain the Doppler effect.

[ 2 ]

(b)

A car is moving at constant velocity v between two sources of sound as shown in the diagram. The sources emit sound at 440 Hz and at 540 Hz . The driver hears both sounds at the same frequency. The speed of sound is $340 \mathrm{~ms}^{-1}$ .

Determine v.

[ 2 ]

(c)

The spectrum of light received from a galaxy shows a peak at a wavelength of 659.0 nm . The peak corresponds to an atomic transition in the hydrogen atom that emits a wavelength of 656.1 nm when observed in a laboratory on Earth.

Calculate the velocity of the galaxy with respect to Earth.

[ 2 ]

[Maximum number: 3]

A sound detector moves along a line connecting it to a stationary loudspeaker.

The loudspeaker emits a sound of frequency 1700 Hz . The speed of sound in air is $340 \mathrm{~ms}^{-1}$ . The detected frequency of the sound is 1600 Hz .

(a)

State the direction of motion of the detector.

[ 1 ]

(b)

Calculate the speed of the detector.

The loudspeaker and the detector are now stationary and above a surface of water. The loudspeaker is at point L and the detector is at point D. L and D are at the same height above the surface of the water. The sound reaches D by two routes: along the direct path LD and the reflection-path LPD.

The following data are given:

\begin{aligned} \text { Frequency of the sound wave } & =1700 \mathrm{~Hz} \\ \text { Speed of sound in air } & =340 \mathrm{~m} \mathrm{~s}^{-1} \\ \text { Speed of sound in water } & =1500 \mathrm{~m} \mathrm{~s}^{-1} \\ \text { Distance LD } & =0.70 \mathrm{~m} \\ \text { Distance LP } & =0.50 \mathrm{~m} \end{aligned}

[ 2 ]

[Maximum number: 1]

A source S of a sound wave is moving at a constant velocity along a line joining stationary observers X and Y . The diagram shows consecutive wavefronts of the sound.

Consider the following statements:

I. More wavefronts pass X in a unit time than Y .
II. The speed of the wavefronts is greater at X than at Y .
III. The wavelength of the sound at X is less than the wavelength at Y .

Which statements are correct?

I and II only

I and III only

II and III only

I, II and III

[Maximum number: 5]

The following data are available for the Sun when it entered the main sequence.

\begin{aligned} \text { Mass } & =2.0 \times 10^{30} \mathrm{~kg} \\ \text { Radius } & =7.0 \times 10^{8} \mathrm{~m} \\ \text { Surface temperature } & =5800 \mathrm{~K} \\ \text { Core temperature } & =1.5 \times 10^{7} \mathrm{~K} \\ \text { Core density } & =1.6 \times 10^{5} \mathrm{~kg} \mathrm{~m}^{-3} \end{aligned}

(a)

The Sun rotates about its axis. P is a point on the Sun's equator.

A particular spectral line of hydrogen from a laboratory source has wavelength 656.2797 nm . The same spectral line emitted from P has wavelength 656.2753 nm when measured on Earth.

[ 5 ]

(i)

Explain this observation.

[ 2 ]

(ii)

Calculate the period of revolution of the Sun.

[ 3 ]

[Maximum number: 4]

Star A is a main sequence star.

The data for Star A and the Sun are given in a table.

(a)

The peak wavelength of the radiation from Star A is measured by an observer on Earth. The observed wavelength is 0.007 % less than the value in (c)(i).

Outline how this difference arises.

[ 2 ]

(b)

Determine the speed, in $\mathrm{km} \mathrm{s}^{-1}$ , of Star A relative to Earth.

[ 2 ]

[Maximum number: 4]

Star A is a main sequence star.

The data for Star A and the Sun are given in a table.

(a)

The peak wavelength of the radiation from Star A is measured by an observer on Earth. The observed wavelength is 0.007 % less than the value in (c)(i).

Outline how this difference arises.

[ 2 ]

(b)

Determine the speed, in $\mathrm{km} \mathrm{s}^{-1}$ , of Star A relative to Earth.

[ 2 ]

0 selected

Exam points

Question 16

Question 4

Question 4(c)(i)

Question 4(c)(ii)

Question 4(c)(iii)

Question 5

Question 5(a)(i)

Question 5(a)(ii)

Question 23

Question 5

Question 5(e)

Question 5(e)(i)

Question 5(e)(ii)

Question 5

Question 5(c)(iii)

Question 5(c)(iv)

Question 7

Question 7(c)(iii)

Question 7(c)(iv)