AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Since a good absorber is also a good emitter, a perfect black body would…
Full specification wording
Emission and absorption of infrared radiation: Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.
A surface that absorbs a given wavelength well also emits that wavelength well, at the same temperature.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A surface that absorbs a given wavelength well also emits that wavelength well, at the same temperature. If that were not so, an object sitting in a room at constant temperature would spontaneously warm or cool, which never happens. So the perfect absorber must also be the perfect emitter: a perfect black body is the best possible emitter of radiation, which is the statement examiners ask you to select. The laboratory evidence runs both ways. A matt black flask cools fastest because it emits best, and a matt black cube heats fastest because it absorbs best, while silvered surfaces are poor at both, which is exactly why a vacuum flask is silvered inside.
What earns the marks
- Stating that a good absorber of radiation is also a good emitter.
- Concluding that a perfect black body is the best possible emitter of radiation.
- Stating that the comparison of absorbers and emitters must be made at the same temperature.
- Applying the idea to a named surface, such as a matt black flask cooling faster than a silvered one.
Common mistakes and how to put them right
Assuming a good absorber must be a poor emitter because it keeps the energy it takes in.
Saying a black surface emits more visible light rather than more infrared.
Explaining a silvered flask only by reflection inwards, without saying silver is also a poor emitter.
Reading 'best emitter' as meaning the body must be hot.
Ticking 'they are good reflectors of radiation' or 'they easily transmit radiation' as the true statement about perfect black bodies.
Exam tips
- Put absorber and emitter in the same sentence so the link is explicit.
- Use the flask cooling experiment and the cube heating experiment as your two pieces of evidence.
- Take the comparison to the same temperature, or the comparison is not fair.
Try the lesson
The start of the Emission and absorption of infrared radiation: Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter. lesson, made from checked teaching notes for this specification point.
Have a go first
A perfect black body is a very good absorber of radiation. What does this tell you about its ability to emit radiation?
Listen
A surface that absorbs a given wavelength well also emits that wavelength well, at the same temperature. If that were not so, an object sitting in a room at constant temperature would spontaneously warm or cool, which never happens. So the perfect absorber must also be the perfect emitter: a perfect black body is the best possible emitter of radiation, which is the statement examiners ask you to select. The laboratory evidence runs both ways. A matt black flask cools fastest because it emits best, and a matt black cube heats fastest because it absorbs best, while silvered surfaces are poor at both, which is exactly why a vacuum flask is silvered inside. A strong answer earns marks for these points. stating that a good absorber of radiation is also a good emitter. concluding that a perfect black body is the best possible emitter of radiation. stating that the comparison of absorbers and emitters must be made at the same temperature. applying the idea to a named surface, such as a matt black flask cooling faster than a silvered one. Exam tip: Put absorber and emitter in the same sentence so the link is explicit.
See it
Key words
Say what each word means before you turn the card.
good absorber is a good emitter
Try answering aloud or in your head before turning the card.
black body
Try answering aloud or in your head before turning the card.
silvered surfaces
Try answering aloud or in your head before turning the card.
See every definition
- good absorber is a good emitter
- A surface that absorbs a given wavelength well also emits that wavelength well, at the same temperature.
- black body
- The perfect absorber, which must also be the perfect emitter and so is the best possible emitter of radiation.
- silvered surfaces
- Surfaces that are poor at both absorbing and emitting, which is exactly why a vacuum flask is silvered inside.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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