Theme C — Wave Behaviour
C.5 Doppler Effect — IB Physics HL Revision Notes & Topical Questions
5 min read 11 parts Lesson 33 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 Doppler Effect — IB PHYSICS HL notes: Explain the Doppler effect for sound and light, use Δf/f = Δλ/λ ≈ v/c, interpret redshift and apply the Doppler equations for sound.Definition — the change in observed frequency due to relative motion between a source of sound or light an…
- Stationary
- Source frequency
- Approaching
- Wave speed
- Wavefronts ahead
- Wavefronts behind
Part 1 of 9The Doppler effect
Definition
- Definition the change in observed frequency due to relative motion between a source of sound or light and an observer
Stationary
- Stationary a stationary source and observer have the same frequency and wavelength
Source frequency
- Source frequency the source itself keeps a constant frequency
Approaching
- Approaching higher observed frequency and shorter wavelength
Receding
- Receding lower observed frequency and longer wavelength
Wave speed
- Wave speed it is unchanged in the medium
Part 2 of 9Moving source
Wavefronts ahead
- Wavefronts ahead bunched together, so a shorter wavelength and higher frequency
Wavefronts behind
- Wavefronts behind spread out, so a longer wavelength and lower frequency
Size of shift
- Size of shift the bigger the change in wavelength, the bigger the Doppler shift
Example
- Example an ambulance siren sounds higher approaching and lower after passing
Part 3 of 9Moving observer
Wavelength in the medium
- Wavelength in the medium unchanged
Observer approaching
- Observer approaching meets wavefronts more often, so the observed frequency is higher
Observer receding
- Observer receding meets wavefronts less often, so the observed frequency is lower
Part 4 of 9Doppler effect of light
Red shift
- Red shift light from a source moving away shifts to longer wavelengths
- It is like sound dropping in frequency
Blue shift
- Blue shift light from a source moving towards the observer shifts to shorter wavelengths
Relation
- Δf/f ≈ Δλ/λ ≈ v/c, valid only for v much less than c
Wavelength change
- Δλ = λobserved − λreference
Relative velocity
- Relative velocity v = vs − vo along the line joining source and observer
Dimension
- Dimension the ratio has no unit
Part 5 of 9Spectral lines
Spectral lines
Characteristic wavelengths
- Characteristic wavelengths each element absorbs and emits at fixed wavelengths
Comparison
- Comparison compare a star's lines with laboratory values to find Δλ
Red shift
- Red shift lines shifted to longer wavelength show the source receding
Rotation
- Rotation opposite shifts on the two sides of a star or galaxy reveal its rotation, with rotational speed from half the difference in wavelength
Method
- Method find Δλ, then v = cΔλ/λ
Part 6 of 9Galactic redshift
Redshift
- Redshift the fractional increase in wavelength due to source and observer receding
Evidence
- Evidence distant galaxies are redshifted compared with the Sun, showing that the universe is expanding
Sign of velocity
- Sign of velocity a negative relative velocity means the galaxy is moving away, with observed frequency less than the reference frequency
Balloon model
- Balloon model as space stretches, galaxies move apart like dots on an inflating balloon
Distance
- Distance the further the galaxy, the greater its redshift and recession speed
Part 7 of 9Applications
Radar speed guns
- Radar speed guns measure the frequency shift of microwaves reflected from a vehicle
Medical ultrasound
- Medical ultrasound measures blood flow speed from the shift in reflected ultrasound
Astronomy
- Astronomy measures rotation and recession of stars and galaxies
Part 8 of 9Doppler equations for sound (HL)
Moving source
- Moving source f′ = f v/(v ± us)
- Approachingv − us
- Recedingv + us
Moving observer
- Moving observer f′ = f (v ± uo)/v
- Approachingv + uo
- Recedingv − uo
Symbols
- Symbols v is the speed of sound, about 340 m s−1, us is the source speed and uo the observer speed
Part 9 of 9More from the specification
1Astrophysical Doppler shifts observed on Earth
- Astrophysical Doppler shifts observed on Earth detecting cosmological redshift in light from distant galaxies received on Earth confirms universe expansion.
2Doppler effect
- Doppler effect a change in observed frequency or wavelength when the source and observer move relative to each other
3Moving source
- Moving source wavefronts bunch up in front, giving an increase in frequency, and spread behind, giving a decrease
4Sound formula
- Sound formula f' = f v/ , with the sign depending on whether the source approaches or recedes
- (v plus or minus vs)
5Electromagnetic waves
- Electromagnetic waves for light, the shift in wavelength gives /lambda = v/c for speeds much less than c
- (change in lambda)
6Galactic redshift
- Galactic redshift spectral lines from receding galaxies shift toward longer wavelengths, showing the Universe is expanding
7Spectrum
- Range of colours in white light.
8Electromagnetic wave
- Transverse wave that transfers energy from a source to an absorber
9Galactic redshift
- Shift of galaxy spectral lines to longer wavelength due to recession.
10Spectral lines
- Dark or bright lines in a spectrum at wavelengths characteristic of elements.
Key terms
| Term | Meaning |
|---|---|
| Doppler effect | The change in observed frequency of a wave due to relative motion between source and observer. |
| Red shift | An increase in observed wavelength of light from a receding source. |
| Blue shift | A decrease in observed wavelength of light from an approaching source. |
| Spectral line | A discrete wavelength emitted or absorbed by an element. |
| Redshift (z) | The fractional increase in wavelength due to the source and observer receding. |
Avoid these mistakes
1
Saying the source frequency changes — only the observed frequency changes.
2
Treating red shift as meaning the star is approaching — it means recession.
3
Using the Δλ/λ ≈ v/c relation for speeds close to c.
4
Using the wrong sign in the sound equations — approach must give f′ > f.
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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.