Emission and Absorption of Infrared Radiation
Get to grips with Emission and Absorption of Infrared Radiation for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Investigate how the infrared radiation emitted or absorbed by a surface depends on the nature of that surface.
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Topic overview
All objects emit and absorb infrared radiation. The hotter an object is, the more infrared radiation it radiates in a given time. This is a required practical.
Dark, matt surfaces are good emitters and good absorbers of infrared radiation. Light, shiny surfaces are poor emitters and poor absorbers, and they are good reflectors.
The practical uses a Leslie cube — a metal cube with faces of different surface finishes, filled with hot water. An infrared detector measures the radiation emitted from each face at the same distance. The matt black face emits the most and the shiny silver face the least.
Revision notes
Emission and absorption
All objects emit and absorb infrared radiation, and the hotter an object is the more it radiates in a given time.
Dark matt surfaces are good emitters and good absorbers. Light shiny surfaces are poor emitters and poor absorbers, and are good reflectors.
The Leslie cube
A Leslie cube is a metal cube with faces of different finishes — typically matt black, shiny black, matt white and shiny silver.
It is filled with hot water, and an infrared detector measures the radiation from each face at the same distance.
Control variables
The detector must be the same distance from each face, and the water temperature must be the same for each measurement.
Without these controls the comparison between surfaces would not be valid. Results should be repeated and a mean taken.
Key points
- All objects emit and absorb infrared radiation.
- Hotter objects radiate more in a given time.
- Matt black surfaces are good emitters and absorbers.
- Shiny silver surfaces are poor emitters and absorbers.
- A Leslie cube compares different surfaces.
- Keep the detector at the same distance each time.
Worked examples
Example 1
State which surface is the best emitter of infrared radiation. [1 mark]
Model answer
A matt black surface (1 mark).
Example 2
Explain why the infrared detector must be the same distance from each face of the Leslie cube. [2 marks]
Model answer
The intensity of infrared radiation decreases with distance from the source (1 mark), so keeping the distance constant ensures that any difference measured is caused by the surface finish rather than the distance (1 mark).
Example 3
Explain why a shiny silver surface is a poor absorber of infrared radiation. [2 marks]
Model answer
A shiny surface reflects most of the infrared radiation falling on it (1 mark), so little is absorbed and the object heats up slowly (1 mark).
Common mistakes
Saying only hot objects emit infrared.
All objects emit it; hotter ones simply emit more in a given time.
Saying shiny surfaces are good absorbers.
They are poor absorbers and good reflectors.
Not controlling the detector distance.
Intensity falls with distance, so this must be kept constant.
Confusing emitter and reflector.
Good emitters are good absorbers; good reflectors are poor at both.
Exam tips
- Remember good emitters are also good absorbers.
- Name the detector distance as a control variable.
- Link shiny surfaces to reflection.
- State that all objects emit infrared, not just hot ones.
Key terms
- Leslie cube
- A cube with different surface finishes for comparing emission.
- Matt
- A dull, non-reflective surface finish.
- Emitter
- An object radiating infrared.
- Absorber
- An object taking in infrared radiation.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.