Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

8.3.1The Periodic Table

The Doppler Effect — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 16 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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
0%read

Part 1 of 4What the Doppler Effect Is

8.3.1 The Doppler Effect
Doppler effect
  • The change in the observed wavelength and frequency of a wave when the source of the wave moves relative to the observer.
Wave types
  • The effect applies to sound, light and microwaves, for any type of wave from a moving source.
Police siren
  • The pitch of a siren changes as a moving car passes, because the observed wave pattern changes.
Red-shift
  • Light from distant galaxies is observed at longer wavelengths because those galaxies are moving away from us.

Part 2 of 4Source Moving Towards or Away

1
When a source moves towards an observer, the waves bunch together, so the observed wavelength decreases and the frequency increases.
2
When a source moves away from an observer, the waves stretch out, so the observed wavelength increases and the frequency decreases.
3
The speed of the emitted waves does not change, so a change in observed wavelength always means a change in observed frequency.
4
An observer behind a moving source therefore sees stretched waves with a lower observed frequency.

Part 3 of 4Positions of the Observer

Positions of the Observer
In front
  • A stationary observer in front of a moving source detects a higher frequency than the emitted frequency.
Behind
  • A stationary observer behind a moving source detects a lower frequency than the emitted frequency.
Wave speed
  • The speed of the waves through the medium stays the same, so only the observed wavelength and frequency change.
Stationary source
  • A stationary source emits waves symmetrically in all directions, all with the same wavelength.

Part 4 of 4Exam Checklist & Tips

8.3.1 – Key Points for the Exam

  • Doppler effect An apparent change in wave frequency or wavelength caused by relative motion between source and observer.
  • Observed frequency The frequency of a wave as it is detected by an observer.
  • Red-shift An increase in the observed wavelength of light, moving it towards the red end of the spectrum.
  • Blue-shift A decrease in the observed wavelength of light, moving it towards the blue end of the spectrum.
  • Wavelength The distance between two consecutive points in phase on a wave.

Common Mistakes

  • Saying that the waves themselves change speed; the apparent change comes from the motion of the source.
  • Mixing up approaching and receding sourcesan approaching source has a higher observed frequency.
  • Stating that the Doppler effect applies only to sound; it applies equally to light and microwaves.

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.