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Theme C — Wave Behaviour

C.5 Doppler Effect — IB Physics SL Revision Notes & Topical Questions

5 min read 7 parts Lesson 28 of 31 Unit quiz

📚 IB Physics SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

C.5 Doppler Effect — IB PHYSICS SL notes: Explain the Doppler effect for sound and light, represent it with wavefront diagrams and use spectral line shifts to find the speeds of stars and galaxies.Doppler effect — the change in observed frequency of a wave due to relative motion …

  • Approaching
  • Moving source
  • Moving observer
  • Relative velocity
  • Blue shift
  • Radar speed guns

Part 1 of 5The Doppler effect

Doppler effect
  • Doppler effect the change in observed frequency of a wave due to relative motion between source and observer
  • The frequency emitted by the source does not change.
Approaching
  • Approaching the observed frequency is higher and the wavelength shorter
Receding
  • Receding the observed frequency is lower and the wavelength longer
FeatureApproachingReceding
Observed frequencyHigherLower
Observed wavelengthShorterLonger
Light shiftBlue shiftRed shift

Part 2 of 5Moving source and moving observer

Moving source
  • Moving source wavefronts ahead of the source are bunched together and behind it they are spread out
    • (shorter wavelength, higher frequency)
  • The wave speed in the medium is unchanged.
Moving observer
  • Moving observer the wavelength in the medium is unchanged,
    • but an observer moving towards the source meets wavefronts more often,
    • so the observed frequency is higher

Part 3 of 5Light

Relative velocity
  • Relative velocity for electromagnetic waves only the relative velocity of source and observer matters
Formula
  • Formula for v much less than c, Δf/f = Δλ/λ ≈ v/c
Red shift
  • Red shift an increase in observed wavelength of light from a receding source
Blue shift
  • Blue shift a decrease in observed wavelength of light from an approaching source

Part 4 of 5Spectral lines

Spectral line
  • Spectral line a discrete wavelength emitted or absorbed by an element
Method
  • Method compare a star's lines with laboratory values to find Δλ
Meaning
  • Meaning lines shifted to longer wavelengths show the source is receding
Rotation
  • Rotation opposite shifts from the two sides of a rotating star or galaxy reveal its rotation
1
Measure the wavelength of a known spectral line from the source.
2
Find Δλ = observed wavelength − laboratory wavelength.
3
Calculate v = cΔλ/λ.
4
Positive Δλ (red shift) means recession and negative Δλ (blue shift) means approach.

Part 5 of 5Other applications

Radar speed guns
  • Radar speed guns they measure the frequency shift of microwaves reflected from a vehicle
Medical ultrasound
  • Medical ultrasound ultrasound reflected from moving blood cells gives the flow speed
Astronomy
  • Astronomy shifts in starlight give the speeds of stars and galaxies

Key terms

TermMeaning
Doppler effectThe change in observed frequency of a wave due to relative motion between source and observer.
Red shiftAn increase in observed wavelength of light from a receding source.
Blue shiftA decrease in observed wavelength of light from an approaching source.
Spectral lineA discrete wavelength emitted or absorbed by an element.

Avoid these mistakes

Avoid these mistakes
Believing the source emits a different frequency as it moves.
Mixing up approach (higher frequency) and recession (lower frequency).
Saying a red shift means the source is approaching.

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.
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