Explain why a perfect black body is considered an ideal emitter as well as an ideal absorber of radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
All objects absorb and emit radiation. Scientists use the concept of a perfect black body as a theoretical model to understand how objects emit and absorb radiation.
Model answer (4 marks)
A perfect black body absorbs all incident radiation, so its absorptivity is 1.
It is an ideal emitter because it emits radiation at the maximum possible intensity for its temperature, across all wavelengths.
The intensity of its emitted infrared radiation increases with temperature (Stefan–Boltzmann law).
When the rate of absorption equals the rate of emission, the black body stays at a constant temperature.
It is an ideal emitter because it emits radiation at the maximum possible intensity for its temperature, across all wavelengths.
The intensity of its emitted infrared radiation increases with temperature (Stefan–Boltzmann law).
When the rate of absorption equals the rate of emission, the black body stays at a constant temperature.
Examiner tips
- Use the term ‘absorptivity = 1’ for full marks on absorption. Mention ‘maximum possible emission’ and ‘all wavelengths’ for the emission part. Include the temperature dependence (Stefan–Boltzmann) to link to the third point. State the equilibrium condition for constant temperature to cover the last point.
Common mistakes
- Saying the black body emits only infrared – it emits across the whole spectrum. Forgetting to mention that absorptivity is 1. Mixing up absorption and emission terms. Not linking the temperature dependence to the amount of radiation emitted.
Mark scheme (4 marks)
- A perfect black body absorbs all radiation incident on it
- A perfect black body emits radiation across all wavelengths / emits the maximum possible radiation for its temperature
- The hotter the object, the greater the amount of infrared radiation emitted in a given time
- A body remains at a constant temperature when it absorbs radiation at the same rate that it emits it
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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