A student claims that a black body radiator emits only visible light. Explain why this claim is incorrect, and describe how the peak wavelength of radiation emitted by a black body changes as its temperature increases.

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

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

A black body is a perfect absorber and emitter of all wavelengths of electromagnetic radiation. The wavelength at which most radiation is emitted depends on the temperature of the object.

Model answer (4 marks)

A black body emits radiation across the whole electromagnetic spectrum, not just visible light.

The peak wavelength of the spectrum is given by Wien’s law: λ_max = 2.9×10^−3 m·K / T.

As the temperature T rises, λ_max decreases.

At low temperatures the peak lies in the infrared; as the temperature increases the peak moves into the visible range (red, then orange, yellow, white, blue‑white).

Examiner tips

  • Use the phrase "across the whole electromagnetic spectrum" to show understanding of all wavelengths.
  • State Wien’s law or explain that λ_max decreases as T increases.
  • Give examples of colour shift with temperature.
  • Mention infrared at low T and visible at high T.

Common mistakes

  • Claiming the black body emits only visible light.
  • Ignoring the infrared part of the spectrum.
  • Not linking the shift of λ_max to temperature increase.

Mark scheme (4 marks)

  1. A black body emits radiation across all/a range of wavelengths (not just visible light)
  2. The peak wavelength decreases as temperature increases
  3. At lower temperatures, the peak wavelength is in the infrared region
  4. At higher temperatures, the peak wavelength shifts into the visible region (e.g. red, then white/blue-white light)

Key terms in this question

black body radiator · peak wavelength

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