Redshift is observed in light from distant galaxies. Describe what redshift is and explain what it tells us about how galaxies are moving, including how the speed of recession relates to distance.

Cambridge International IGCSE Physics (0625) — 6.2 Stars and the Universe · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Redshift is an increase in the observed wavelength of light emitted from a galaxy.
It occurs because the galaxy is moving away from Earth.
Galaxies that are further away show greater redshift, meaning they are receding faster.
This evidence supports the idea that the Universe is expanding.

Examiner tips

  • Use the term ‘redshift’ and ‘increase in wavelength’. Explain the cause as recession. Mention the distance–speed relationship. Link to the expanding universe conclusion.

Common mistakes

  • Confusing redshift with blueshift. Saying galaxies are moving towards us. Omitting the distance–speed correlation.

Mark scheme (4 marks)

  1. Redshift is an increase in the observed wavelength of electromagnetic radiation (light) emitted from a galaxy
  2. Redshift occurs because the galaxy is receding / moving away from Earth
  3. Galaxies that are further away show greater redshift / are receding faster
  4. This evidence supports the idea that the Universe is expanding

Key terms in this question

redshift

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