IB Physics SL C.5 Doppler effect Questions

SLSL HLHL

Trace sound-frequency changes to source motion, read stellar spectral shifts to infer direction, and use galaxy red-shift with Hubble’s law to estimate distance.

22 questions in this syllabus scope, with question parts counted together.

Exam points & syllabus

Exam points

Define the Doppler effect using both observed frequency or pitch change and relative motion between the source and observer.
Explain approach and recession through wavefront spacing and arrival rate: closer fronts arrive more often; wider fronts arrive less often.
Construct moving-source circular fronts about successive emission positions, using the stated source direction, speed and grid to shift the centres.
Use the sign and endpoint of a frequency-shift graph: an approaching source that stops without passing has a positive shift that falls to zero.
Compare corresponding emitted or laboratory and received spectral features to identify approach or recession; specify the direction before its mechanism.
Find source speed from the fractional light-shift magnitude, using the emitted reference wavelength or frequency rather than the received value.
When calculating received wavelength, distinguish the shift from the final value; for recession add Δλ to the emitted wavelength.
For a receding radio source specified by emitted frequency, reduce the frequency first and then obtain the received wavelength from λ′=c/f′.
Convert percent spectral shift to a fraction before multiplying by c, then express speed in the requested units using the light-wave relation.
For fixed emitted wavelength at low relative speed, recognise Δλ versus recession speed as a positive straight line through the origin.
Infer spectral velocity by measuring a line’s fractional shift against its reference, then applying the light Doppler relation.
Follow line-of-sight motion through the supplied binary orbit before shifting each spectral line blueward or redward from its original position.

Covered syllabus points

C.5.1—Doppler effect C.5.2—Doppler wavefront diagrams C.5.3—Light Doppler approximation C.5.4—Spectral line shifts

[Maximum number: 2]

This question is about the Doppler effect.

Georgia carries out an experiment to measure the speed of mosquitoes. She sets up a microphone to record the sounds of passing mosquitoes.

One mosquito is moving in a straight line with constant speed and passes very close to the microphone as seen in the diagram. The mosquito produces a sound of constant frequency. The speed of sound in air is $340 \mathrm{~m} \mathrm{~s}^{-1}$ .

The maximum frequency recorded is 751 Hz and the minimum frequency recorded is 749 Hz . Explain this observation.

[Maximum number: 2]

This question is about the Doppler effect.

The diagram shows wavefronts in air produced by a stationary source S of sound.
The distance between successive wavefronts is equal to the wavelength of the sound.
The speed of sound is c.

On the diagram, sketch three successive wavefronts produced when S is moving to the left at a speed of 0.5 c.

[Maximum number: 2]

Star A is a main sequence star.

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

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

$\begin{aligned} v=\frac{\Delta \lambda}{\lambda} c=\frac{0.007}{100} \times 3 \times 10^{8}=21000 21 \mathrm{~km} \mathrm{~s}^{-1} \end{aligned}$

Allow ECF for MP2.

Do not award MP1 for use of the Doppler formulas for sound and mechanical waves.

[Maximum number: 1]

The Hubble constant is $2.3 \times 10^{-18} \mathrm{~s}^{-1}$ .

Outline how observations of spectra from distant galaxies provide evidence that the universe is expanding.

Question 1

Question 1

Question 1

Question 1