Emission and absorption of infrared radiation — AQA GCSE Physics
Test yourself on Emission and absorption of infrared radiation with AQA GCSE practice questions.
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Emission and absorption of infrared radiation explained
Every object emits infrared radiation all the time, whatever its temperature, because its particles have internal energy; even ice emits infrared.
Read the full explanation
At the same moment it is absorbing infrared coming from its surroundings, so emission and absorption always happen together and it is the balance between them that decides whether the temperature changes. How well a surface does either depends on its colour and its texture. In the AQA cube experiment four cubes of the same size were placed the same distance from an infrared heater: matt black rose 6.5 °C, shiny black 4.0 °C, matt white 3.0 °C and shiny white only 2.0 °C, showing that black absorbs more than white and that matt absorbs more than shiny of the same colour.
The hotter the body, the more infrared radiation it radiates in a given time.
The rate at which a body radiates infrared rises steeply as its temperature rises, so a hot object loses energy by radiation quickly at first and more slowly as it cools. That is why a cooling curve is steepest at the start and flattens off towards room temperature. In the flask experiment the flasks emit at the greatest rate during the first minute, because that is when the water is hottest and the temperature difference between the flask and its surroundings is greatest. Cooling never stops abruptly; once the flask reaches the temperature of its surroundings it is still emitting, but at the same rate as it absorbs. Rate of emission is not the same as total energy emitted over the whole experiment.
A perfect black body is an object that absorbs all of the radiation incident on it.
A perfect black body is an idealised object that absorbs every bit of radiation that lands on it, at every wavelength. Nothing is reflected and nothing is transmitted, so the absorbed fraction is one. No real object is perfect, but a matt black surface comes close, and so does a small hole leading into a large cavity, because radiation entering the hole bounces around inside until it is absorbed. Stars are treated as black bodies, which shows that the name is about absorbing rather than about looking black: a star absorbs everything incident on it and is dazzling only because it is emitting so strongly at its own temperature. A perfect absorber is also the best possible emitter.
A black body does not reflect or transmit any radiation.
Radiation arriving at a surface can only do three things: be reflected, be transmitted, or be absorbed. A perfect black body does neither of the first two, so the whole of the incident radiation must be absorbed, and that is what makes the model so simple to work with. Real surfaces fall short in one of those two ways: shiny silver reflects most of the infrared that reaches it, which is why a vacuum flask is silvered, while clear glass transmits most of the visible light that reaches it. The shiny white cube rose only 2.0 °C in the AQA experiment because so much was reflected. It is the absence of reflection that makes a cold black body look black in the first place.
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. 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.
Your focus
- State the condition under which an object emits infrared radiation.
- Conclude from a table of temperature rises which surface is the best absorber, comparing like with like.
- Explain why an object can be absorbing and emitting infrared radiation at the same time.
Show all 15 objectives
- Identify from a cooling curve when a body radiates at the greatest rate and justify the choice.
- Explain why the rate of infrared emission falls as an object cools towards room temperature.
- Distinguish the rate of emission from the total energy emitted in a stated interval.
- Define a perfect black body in terms of absorbed, reflected and transmitted radiation.
- Explain why a star is modelled as a black body even though it is bright.
- Identify which of a set of surfaces is the closest approximation to a black body and justify the choice.
- Deduce what fraction of the radiation falling on a perfect black body is absorbed, given that none is reflected and none is transmitted.
- Reject the claim that black bodies are good reflectors and justify the rejection.
- Explain why a matt black surface is a better model of a black body than a shiny black one.
- Use the cooling results for a matt black flask and a silvered flask to show that the better absorber is also the better emitter.
- Justify the statement that a perfect black body is the best possible emitter.
- Predict which of two flasks cools faster and support the prediction with the absorber and emitter rule.
Emission and absorption of infrared radiation exam tips
Marking Points
- one mark for stating that all objects emit infrared radiation whatever their temperature
- one mark for stating that objects absorb infrared radiation from their surroundings at the same time
- one mark for concluding that black surfaces absorb more infrared than white surfaces
- one mark for concluding that matt surfaces absorb more infrared than shiny surfaces of the same colour
- one mark for identifying the earliest time interval as the one with the greatest rate of emission
- one mark for the reason that the water is hottest then, so the temperature difference with the surroundings is greatest
- one mark for stating that the rate of energy transfer by radiation increases as temperature increases
- one mark for linking a steeper section of a cooling curve to a greater rate of emission
- one mark for defining a perfect black body as an object that absorbs all the radiation incident on it
- one mark for stating that none of the incident radiation is reflected
- one mark for stating that none of the incident radiation is transmitted
- one mark for naming a good approximation to a black body, such as a matt black surface or a star
- one mark for stating that a black body reflects none of the incident radiation
- one mark for stating that a black body transmits none of the incident radiation
- one mark for concluding that all of the incident radiation is therefore absorbed
- one mark for identifying a shiny or transparent surface as a poor approximation to a black body and saying why
- one mark for stating that a good absorber of radiation is also a good emitter
- one mark for concluding that a perfect black body is the best possible emitter of radiation
- one mark for stating that the comparison of absorbers and emitters must be made at the same temperature
- one mark for applying the idea to a named surface, such as a matt black flask cooling faster than a silvered one
Examiner Tips
- 💡Give two separate conclusions when asked for two: one about colour, one about finish.
- 💡Quote the temperature figures as support for each conclusion.
- 💡Keep the control variables in mind: same size cube, same distance, same time.
- 💡Get the tick right first: the reason mark usually depends on choosing the correct interval.
- 💡Use the phrase 'temperature difference between the object and its surroundings'.
- 💡On a cooling graph, read the gradient, not the temperature, when a question asks about rate.
- 💡Build the definition from three ideas: absorbs all, reflects none, transmits none.
- 💡If a question says stars act like black bodies, use it as the example rather than fighting it.
- 💡Use 'radiation' rather than 'light' so the definition covers all wavelengths.
- 💡In tick-one questions, rule out the reflection and transmission options straight away.
- 💡Use a silvered vacuum flask as the worked counter-example of a poor absorber.
- 💡Say 'radiation' not 'heat' when writing about what the body does or does not do.
- 💡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.
Common Mistakes
- writing a conclusion such as 'black absorbs more' without saying more than what
- stating that only hot objects emit infrared radiation
- saying cold objects absorb radiation but never emit any
- treating the measured temperature increase as the radiation the cube emitted rather than absorbed
- choosing the last minute because the flask has been emitting for longest
- giving the temperature alone as the reason rather than the difference from the surroundings
- saying the flask stops emitting once it reaches room temperature
- confusing the rate of emission with the total energy emitted during the experiment
- restricting the definition to visible light rather than radiation of every wavelength
- insisting that a black body must look black even when it is very hot
- assuming that absorbing everything means the temperature must rise without limit
- naming a shiny black surface as a perfect black body
- selecting 'they are good reflectors of radiation' or 'they easily transmit radiation' in a multiple choice item
- allowing a perfect black body to reflect a small amount
- concluding that a black body emits nothing, when it is in fact the best emitter
- assuming the definition means the body can never reach a steady temperature
- 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