Explain how astronomers use the Doppler effect to determine whether a distant star is moving towards or away from Earth, and why the magnitude of the observed frequency shift can be used to estimate the star's speed relative to Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Astronomers observe light from distant stars using spectrometers. The spectrum of a star contains dark absorption lines at characteristic frequencies corresponding to known elements such as hydrogen.
Model answer (4 marks)
Astronomers compare the frequency (or wavelength) of the star’s absorption lines with the laboratory rest frequency (or wavelength) of the same lines.
1. If the star is receding, the observed frequency is lower (red‑shifted) and the wavelength longer.
2. If the star is approaching, the observed frequency is higher (blue‑shifted) and the wavelength shorter.
3. The Doppler relation
Δf/f = -v/c (or Δλ/λ = v/c)
links the fractional shift to the star’s speed v along the line of sight; a larger shift gives a larger v.
4. Only the component of the velocity along the line of sight (radial velocity) produces the shift, so the method measures radial speed only.
1. If the star is receding, the observed frequency is lower (red‑shifted) and the wavelength longer.
2. If the star is approaching, the observed frequency is higher (blue‑shifted) and the wavelength shorter.
3. The Doppler relation
Δf/f = -v/c (or Δλ/λ = v/c)
links the fractional shift to the star’s speed v along the line of sight; a larger shift gives a larger v.
4. Only the component of the velocity along the line of sight (radial velocity) produces the shift, so the method measures radial speed only.
Examiner tips
- Use the rest frequency as a reference point; state the shift direction (red/blue).
- Show the Doppler equation and explain the sign convention.
- Mention that only radial velocity contributes to the shift.
- Keep the answer concise and use the exact terminology (red‑shift, blue‑shift, radial velocity).
Common mistakes
- Confusing red‑shift with blue‑shift or vice versa.
- Using the wrong sign in the Doppler equation.
- Failing to note that only the radial component of velocity is measured.
Mark scheme (4 marks)
- The observed frequency (or wavelength) of absorption lines is compared with the known rest frequency (or wavelength) of those lines measured in a laboratory.
- If the star moves away from Earth, the observed frequency is lower (redshift); if it moves towards Earth, the observed frequency is higher (blueshift).
- The Doppler equation relates the fractional frequency shift Δf/f (or Δλ/λ) to the ratio of the source's speed to the speed of light, so a larger shift corresponds to a greater speed.
- Only the component of the star's velocity along the line of sight (radial velocity) contributes to the Doppler shift, so the method measures radial speed only.
Key terms in this question
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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