Explain why objects that behave like a black body are considered ideal emitters and absorbers of radiation, and describe how the temperature of a black body affects the infrared radiation it emits.

WJEC A-Level Physics (Wales) — 6.3 Stellar evolution and cosmology (A-Level only) · Explain · 5 marks · View as Markdown

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

Scientists use the idea of a black body to model how stars and other hot objects emit radiation across the electromagnetic spectrum.

Model answer (5 marks)

A black body is an ideal emitter and absorber because it absorbs all incident radiation – none is reflected or transmitted – and it emits the maximum possible radiation at a given temperature. Consequently, at a constant temperature a black body radiates the same average power that it absorbs. The temperature of a black body determines the intensity of its infrared emission: the hotter the body, the greater the infrared radiation it emits in a given time. Since all objects emit and absorb infrared radiation, a black body provides a useful model for any object that absorbs and re‑emits radiation, such as a star.

Examiner tips

  • Use the phrase "absorbs all incident radiation" and "emits the maximum possible radiation" to hit the key points.
  • Mention the equality of absorbed and emitted power at constant temperature.
  • Explain the temperature–radiation relationship with the word "hotter" and "greater infrared radiation".
  • Show that a black body is a useful model for stars by linking to their emission and absorption of infrared radiation.

Common mistakes

  • Confusing a black body with a perfect reflector – forgetting that it absorbs all radiation.
  • Failing to state that a black body emits the maximum possible radiation at its temperature.
  • Not linking the temperature increase to an increase in infrared radiation.

Mark scheme (5 marks)

  1. A black body absorbs all radiation that is incident on it (no radiation is reflected or transmitted)
  2. A black body is also an ideal emitter of radiation (emits as much radiation as possible at its temperature)
  3. At a constant temperature, a black body radiates the same average power that it absorbs
  4. The hotter the black body, the greater the infrared radiation it emits in a given time
  5. All objects emit and absorb infrared radiation, so a black body is a useful model for any object that absorbs and re-emits radiation (such as a star)

Key terms in this question

black body · infrared radiation · emitter · absorber

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