A distant galaxy is observed to emit electromagnetic radiation. The peak wavelength of this radiation is shifted towards longer wavelengths compared to the same radiation measured from a laboratory source on Earth. Explain what this observation tells astronomers about the galaxy and what it suggests about the universe as a whole.

WJEC A-Level Physics (Wales) — 1.6 Using radiation to investigate stars · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The shift of the peak wavelength to longer values is called a red‑shift.

Red‑shift shows that the galaxy is moving away from us.

The larger the red‑shift, the faster the recession speed – and the further the galaxy is.

Because galaxies in every direction exhibit red‑shift, the whole universe is expanding.

This universal expansion is evidence that the universe started from a single point – the Big Bang.

Examiner tips

  • Use the term "red‑shift" first. Show the link between red‑shift and recession speed. Mention all directions to justify expansion. End with the Big Bang conclusion.

Common mistakes

  • Calling the shift a blue‑shift. Forgetting that red‑shift indicates recession. Not linking red‑shift to the Big Bang.

Mark scheme (5 marks)

  1. The shift towards longer wavelengths is called red-shift
  2. Red-shift indicates the galaxy is moving away from Earth / the observer
  3. The greater the red-shift, the greater the speed at which the galaxy is moving away / the further away the galaxy is, the faster it recedes
  4. Galaxies in all directions show red-shift, suggesting the universe is expanding
  5. This expansion supports the idea that the universe began from a single point / supports the Big Bang theory

Key terms in this question

electromagnetic radiation · peak wavelength · universe

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