Theme C — Wave Behaviour
C.5.1 The Doppler Effect — IB Physics HL Revision Notes & Topical Questions
5 min read 10 parts Lesson 34 of 37 1 quiz
Curriculum Equation Key Wave Speed & Doppler Effect FormulasFast Exam Solution
v = f × λ | f_obs = f × (v ± v_o) / (v ∓ v_s) | T = 1 / f
Lesson summary
C.5.1 The Doppler Effect — IB PHYSICS SL notes: Explain the Doppler effect qualitatively using wavefronts for moving sources and observers, and describe its applications.Doppler effect (Doppler shift) — the change in observed frequency and wavelength of a wave due to relative mot…
- Siren example
- Stationary case
- Which waves
- Wave speed
- Size of the shift
- Approaching a stationary source
Section 1 of 10The effect
The effect
Doppler effect (Doppler shift)
- Doppler effect (Doppler shift) the change in observed frequency and wavelength of a wave due to relative motion between the source and the observer
Siren example
- Siren example an ambulance siren is higher in pitch as it approaches, drops as it passes and is lower as it recedes
- The source emits a constant frequency throughout.
- The rate at which wavefronts reach the observer changes.
Stationary case
- Stationary case when source and observer are both stationary, the frequency is the same for both
Which waves
- Which waves the effect occurs for sound, light and water waves
Medium
- Medium for sound the medium matters, while for light only the relative velocity matters
Section 2 of 10Moving source
Wavefronts
- Wavefronts each spreads as a circle centred on the point where it was emitted, but the source has moved on before emitting the next
In front
- In front the circles are crowded together, so the wavelength is shorter and the observed frequency higher
Behind
- Behind the circles are stretched apart, so the wavelength is longer and the observed frequency lower
Wave speed
- Wave speed it depends only on the medium, so f = v/λ links shorter wavelength to higher frequency
Size of the shift
- Δλ is the change in wavelength, and a bigger change gives a bigger Doppler shift
Section 3 of 10Moving observer
1Approaching a stationary source
- Approaching a stationary source the observer crosses wavefronts more often,
- so the observed frequency rises and the wavelength in the medium is unchanged
2Moving away
- Moving away wavefronts catch the observer less often, so the frequency falls
3Origin
- Origin the relative speed of wave and observer changes (v ± vo) rather than the wavelength
4Formulae
- Formulae the two cases give slightly different formulae for sound, because the medium defines a frame of rest
- For light and for speeds much less than the wave speed the results coincide.
Section 4 of 10Light
Red shift
- Red shift waves moving away from the observer shift towards the red end of the spectrum, like sound appearing lower in frequency
Blue shift
- Blue shift waves moving towards the observer shift towards the blue end, like sound appearing higher in frequency
Reason
- Reason red light has a longer wavelength than blue light
Section 5 of 10Reading wavefront diagrams
Spacing
- Spacing compare the spacing of circles in different directions
- Close spacing ahead of the source means a higher observed frequency for an observer ahead.
- Wide spacing behind means a lower observed frequency for an observer behind.
Passing source
- Passing source only the component of velocity along the line joining source and observer matters, so the frequency changes as the source passes
Perpendicular motion
- Perpendicular motion to first order it produces no shift
Wave equation
- Wave equation v is unchanged, so a longer wavelength means a lower frequency
Section 6 of 10Supersonic sources
Supersonic sources
Speed equal to the wave speed
- Speed equal to the wave speed all wavefronts touch at the front of the source, forming a barrier of compression
Faster than the wave speed
- Faster than the wave speed the wavefronts pile up into a Mach cone, giving a shock wave and a sonic boom
- The half-angle θ of the cone satisfies sin θ = v/us.
Boat analogy
- Boat analogy a boat faster than the water waves leaves a V-shaped bow wave
Section 7 of 10Applications
Police radar
- Police radar waves reflected from a moving car return with a frequency shift proportional to the car's speed
Medical ultrasound
- Medical ultrasound the shift from moving red blood cells gives blood speed, used to detect narrowed arteries and to monitor a fetal heartbeat
Weather radar
- Weather radar the shift from raindrops reveals wind speeds inside storms
Astronomy
- Astronomy the shift of spectral lines in starlight reveals radial velocity towards or away from us
Section 8 of 10More from the specification
More from the specification
Doppler Shift in Electromagnetic Radiation
- The observed frequency and wavelength of electromagnetic radiation shift whenever there is relative radial motion between a radiating source object and an observer on Earth.
Cosmological Redshift of Distant Galaxies
- Light emitted by a distant galaxy is shifted toward longer wavelengths relative to stationary laboratory reference spectra, demonstrating that distant galaxies are moving away from Earth as spacetime expands.
- (redshift, Δλ/λ₀ ≈ v/c)
Galactic Rotation Measurements
- Measuring differential Doppler shifts between opposing edges of a spinning galactic disk allows astrophysicists to determine galactic orbital velocities and map radial velocity curves.
Doppler Shift in Electromagnetic Radiation
- The observed frequency and wavelength of electromagnetic radiation shift whenever there is relative radial motion between a radiating source object and an observer on Earth.
Cosmological Redshift of Distant Galaxies
- Light emitted by a distant galaxy is shifted toward longer wavelengths relative to stationary laboratory reference spectra, demonstrating that distant galaxies are moving away from Earth as spacetime expands.
- (redshift, Δλ/λ₀ ≈ v/c)
Galactic Rotation Measurements
- Measuring differential Doppler shifts between opposing edges of a spinning galactic disk allows astrophysicists to determine galactic orbital velocities and map radial velocity curves.
Galaxy
- Huge group of stars, gas, dust and solar systems held by gravity.
Redshift
- Increase in the wavelength of light from a source moving away from the observer.
Universe
- All of space and everything in it.
Electromagnetic wave
- Transverse wave that transfers energy from a source to an absorber
Galactic redshift
- Shift of galaxy spectral lines to longer wavelength due to recession.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Doppler effect | Change in observed frequency due to relative motion of source and observer. |
| Source frequency | Frequency emitted by the source, fs. |
| Observed frequency | Frequency detected by the observer, fo. |
| Mach cone | Cone of wavefronts from a source moving faster than the wave speed. |
| Red shift | Shift of light to longer wavelength from a receding source. |
| Blue shift | Shift of light to shorter wavelength from an approaching source. |
Section 10 of 10Exam technique
1
Believing the source emits a different frequency as it moves — the emitted frequency is constant.
2
Saying the speed of sound changes with the speed of the source.
3
Mixing up approach (higher frequency) and recession (lower frequency).
4
Forgetting that motion perpendicular to the line joining source and observer gives no first-order shift.
5
Thinking the Doppler effect occurs only for sound.
Marked practiceQuizzes for this lesson
Last updated:
Cambridge 9702 & Edexcel Physics
Mark Scheme Points
Doppler Shift & Galactic Red-shift Criteria
Key Equation: Δλ / λ ≈ Δf / f ≈ v / c
- Red-shift: Fractional increase in observed wavelength from receding galaxies.
- Hubble's Law:
v = H0d(Universe age ≈ 1 / H0). - Evidence for Big Bang: Cosmological red-shift + CMBR at 2.7K.