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8.3.2The Periodic Table

Doppler Shift — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 23 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
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Section 1 of 5The Doppler Effect

A moving wave source changes the wavelength and frequency that an observer detects.

Moving source
  • Moving source When a wave source moves relative to an observer, the observed wavelength and frequency both change.
Approaching source
  • Approaching source Wavefronts are squashed together in front of the source, so the wavelength decreases and the frequency increases.
Receding source
  • Receding source Wavefronts are stretched out behind the source, so the wavelength increases and the frequency decreases.
Light as well as sound
  • Light as well as sound The Doppler effect applies to light, so the spectral lines of moving stars and galaxies are shifted.
Unitless shift
  • Unitless shift The shift is a ratio of two wavelengths and the ratio v / c, so both sides of the equation have no units.

Section 2 of 5Using the Doppler Equation

1
Write the Doppler equation as (λ − λ₀) / λ₀ = v / c, where λ is the observed wavelength and λ₀ is the reference wavelength.
2
Rearrange the equation to find the velocity: v = c × (λ − λ₀) / λ₀.
3
Substitute the observed and reference wavelengths, making sure both are in the same unit, and convert to metres if needed.
4
Evaluate the answer and give it in m/s; a positive value means the source is moving away from the observer.

Section 3 of 5Units, Signs and Terms

Units, Signs and Terms
Units
  • Units Convert nanometres (nm) into metres by multiplying by 10⁻⁹ before substituting into the equation.
Reference wavelength
  • Reference wavelength λ₀ is the wavelength measured in a laboratory, when the source is at rest relative to the observer.
Redshift and blueshift
  • Redshift and blueshift A positive change in wavelength means redshift and recession, while a negative change means blueshift and approach.
Speed of light
  • Speed of light c = 3.0 × 10⁸ m/s is used in the equation, so the calculated speed is in m/s.

Section 4 of 5Worked Example

Question

  • A hydrogen spectral line has a reference wavelength of 656 nm.
  • In the light from a distant galaxy, the same line is observed at 660 nm.
  • Calculate the speed of the galaxy and state whether it is moving towards or away from Earth.
  • (c = 3.0 × 10⁸ m/s)

  1. Step 1: λ₀ = 656 nm and λ = 660 nm, so the change in wavelength is 660 − 656 = 4 nm.
  2. Step 2: Use (λ − λ₀) / λ₀ = v / c, so v = c × (λ − λ₀) / λ₀.
  3. Step 3: Substitute: v = 3.0 × 10⁸ × 4 / 656 = 1.8 × 10⁶ m/s.
  4. Step 4: The observed wavelength is longer, so the light is redshifted and the galaxy is moving away from Earth.

Answer The galaxy is moving away from Earth at about 1.8 × 10⁶ m/s.

Section 5 of 5Exam Checklist & Tips

8.3.2 – Key Points for the Exam

  • Doppler effect The change in observed wavelength or frequency caused by the motion of a wave source relative to an observer.
  • Redshift An increase in observed wavelength of light, caused by a source moving away from the observer.
  • Blueshift A decrease in observed wavelength of light, caused by a source moving towards the observer.
  • Reference wavelength The wavelength of a spectral line measured in a laboratory, with the source at rest.
  • Recession Movement of a source away from the observer.

Common Mistakes

  • Swapping the observed and reference wavelengths, which reverses the sign of the calculated speed.
  • Forgetting to convert nanometres to metres before substituting into the equation.
  • Stating that the wavelength increases for an approaching source; an approaching source is blueshifted.
  • Giving the answer without units or without stating whether the galaxy is moving towards or away from Earth.

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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