Explain why the red-shift observed from a galaxy that is twice as far away as another galaxy is expected to be greater.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Scientists have found that the further away a galaxy is, the faster it appears to be moving away from us.
Model answer (4 marks)
Red‑shift is the stretching of light wavelengths towards the red end of the spectrum when a source moves away from the observer.
A greater red‑shift means the galaxy is receding faster.
Because the recession speed increases with distance, a galaxy twice as far away is moving away twice as fast.
Therefore it will show a larger red‑shift, i.e. a greater increase in wavelength.
A greater red‑shift means the galaxy is receding faster.
Because the recession speed increases with distance, a galaxy twice as far away is moving away twice as fast.
Therefore it will show a larger red‑shift, i.e. a greater increase in wavelength.
Examiner tips
- Use the definition of red‑shift first, then link speed to distance, and finish with the conclusion that the farther galaxy has a greater red‑shift.
- Show the logical sequence: red‑shift → speed → distance → greater red‑shift.
Common mistakes
- Confusing red‑shift with blue‑shift or with Doppler shift of sound.
- Failing to state that speed increases with distance (Hubble’s law).
- Using vague terms like "more" instead of specifying the relationship between distance and red‑shift.
Mark scheme (4 marks)
- Red-shift is the stretching of wavelengths of light (towards longer/redder wavelengths) caused by a source moving away from the observer
- A greater red-shift indicates the galaxy is moving away faster / at a greater speed
- More distant galaxies are moving away from us faster (speed increases with distance)
- Therefore the galaxy twice as far away is moving faster, so produces a greater red-shift / greater increase in wavelength
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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