The Sun and a distant red giant star both emit electromagnetic radiation. The Sun has a surface temperature of approximately 5500 °C, while the red giant has a surface temperature of approximately 3500 °C. Both stars behave as black bodies. Describe how the intensity and peak wavelength of radiation emitted by the Sun differs from that emitted by the red giant, and explain why these differences occur.

Eduqas GCSE Physics — 6.5 Black body radiation · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The Sun emits a higher total intensity (power per unit area) than the red giant.
The Sun’s peak wavelength is shorter than that of the red giant.
A hotter body has more thermal energy, so it radiates more power.
A hotter body’s peak wavelength shifts to the blue (decreases) according to Wien’s law.

Examiner tips

  • Use the words "higher intensity" and "shorter peak wavelength" to match the scheme.
  • Explain the cause in terms of temperature and energy, not just state the fact.
  • Show the relationship to Wien’s law and Stefan‑Boltzmann law briefly.

Mark scheme (4 marks)

  1. The Sun emits radiation of greater (total) intensity / power per unit area than the red giant
  2. The Sun emits radiation at a shorter peak wavelength than the red giant
  3. A hotter body emits more intense radiation / more power because it has more energy to radiate
  4. A hotter body emits radiation with a shorter peak wavelength / peak wavelength decreases as temperature increases

Key terms in this question

intensity · peak wavelength · temperature

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