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
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.
Section 1 of 4What the Doppler Effect Is
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
- Wave types The effect applies to sound, light and microwaves, for any type of wave from a moving source.
Police siren
- Police siren The pitch of a siren changes as a moving car passes, because the observed wave pattern changes.
Red-shift
- Red-shift Light from distant galaxies is observed at longer wavelengths because those galaxies are moving away from us.
Section 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.
Section 3 of 4Positions of the Observer
Positions of the Observer
In front
- In front A stationary observer in front of a moving source detects a higher frequency than the emitted frequency.
Behind
- Behind A stationary observer behind a moving source detects a lower frequency than the emitted frequency.
Wave speed
- Wave speed The speed of the waves through the medium stays the same, so only the observed wavelength and frequency change.
Stationary source
- Stationary source A stationary source emits waves symmetrically in all directions, all with the same wavelength.
Section 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.
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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.