Explain how astronomers use the Doppler effect to determine whether a distant galaxy is moving towards or away from Earth, and how the speed of that galaxy can be estimated from the observation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers analyse the light spectrum emitted by distant galaxies. Each element produces a characteristic set of spectral lines at known, fixed frequencies when measured in a laboratory on Earth.
Model answer (4 marks)
1. Compare the observed frequency (or wavelength) of a spectral line from the galaxy with the known laboratory frequency (or wavelength) of the same line.
2. If the observed frequency is lower (wavelength longer) than the laboratory value, the line is red‑shifted, indicating the galaxy is moving away from Earth.
3. If the observed frequency is higher (wavelength shorter) than the laboratory value, the line is blue‑shifted, indicating the galaxy is moving towards Earth.
4. The fractional change in frequency (Δf/f₀) or wavelength (Δλ/λ₀) is proportional to the galaxy’s radial speed relative to Earth; a larger shift corresponds to a greater speed, which can be calculated using the Doppler formula v = (Δλ/λ₀)c (or v = (Δf/f₀)c).
2. If the observed frequency is lower (wavelength longer) than the laboratory value, the line is red‑shifted, indicating the galaxy is moving away from Earth.
3. If the observed frequency is higher (wavelength shorter) than the laboratory value, the line is blue‑shifted, indicating the galaxy is moving towards Earth.
4. The fractional change in frequency (Δf/f₀) or wavelength (Δλ/λ₀) is proportional to the galaxy’s radial speed relative to Earth; a larger shift corresponds to a greater speed, which can be calculated using the Doppler formula v = (Δλ/λ₀)c (or v = (Δf/f₀)c).
Examiner tips
- Use the exact terms ‘red‑shift’ and ‘blue‑shift’. Show the comparison of observed vs. laboratory values. State the proportionality and give the formula for speed. Keep the answer concise to fit the 4‑mark limit.
Common mistakes
- Confusing red‑shift with blue‑shift. Using the wrong proportionality (e.g. Δλ/λ₀ = v/c instead of Δf/f₀ = -v/c). Forgetting to mention that the shift is measured relative to laboratory values.
Mark scheme (4 marks)
- The observed frequency (or wavelength) of a spectral line from the galaxy is compared to the known laboratory frequency (or wavelength) of the same line.
- If the observed frequency is lower (wavelength longer) than the laboratory value — a red-shift — the galaxy is moving away from Earth.
- If the observed frequency is higher (wavelength shorter) than the laboratory value — a blue-shift — the galaxy is moving towards Earth.
- The fractional change in frequency (or wavelength) is proportional to the speed of the galaxy relative to Earth, so a larger shift corresponds to a greater speed.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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