AQA GCSE Physics · Waves
Infrared radiation required practical
Investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.
A Leslie cube has four different faces and is filled with hot water. An infrared detector held the same distance from each face shows that matt black surfaces emit the most infrared and shiny silver ones the least.
Equipment
- Leslie cube (faces: matt black, shiny black, matt white, shiny silver)
- Kettle of hot water
- Infrared detector (or thermometer with a blackened bulb)
- Ruler
- Heat-resistant mat
Variables
- Independent you change
- Type of surface (colour and texture)
- Dependent you measure
- Infrared radiation detected (detector reading)
- Control you keep the same
- Same distance from each face
- Same water temperature
- Same detector
- Readings taken at the same time after filling
Method
- 1
Put the Leslie cube on a heat-resistant mat and fill it with hot water from a kettle. Put the lid on.
- 2
Wait a minute so all the faces reach the same temperature.
- 3
Hold the infrared detector 10 cm from the matt black face, measured with a ruler, and record the reading.
- 4
Repeat at exactly the same distance for each of the other faces.
- 5
Record the room temperature reading with the detector pointing away from the cube.
- 6
Compare the readings for each surface.
Worked example
Detector held 10 cm from each face. Room reading: 21.0 °C.
| Surface | Detector reading (°C) | Above room (°C) |
|---|---|---|
| Matt black | 41.0 | 20.0 |
| Shiny black | 37.5 | 16.5 |
| Matt white | 32.0 | 11.0 |
| Shiny silver | 26.5 | 5.5 |
Conclusion
The matt black surface emits the most infrared radiation and the shiny silver surface the least, even though all four faces are at the same temperature. Dark, matt surfaces are the best emitters and absorbers of infrared; light, shiny surfaces are the worst and reflect it instead.
Risks
- Hot water can scaldFill the cube on a mat and don’t move it when full.
- The cube’s metal faces get very hotDon’t touch the cube; hold the detector, not the cube.
Common mistakes
- Holding the detector at different distances. Infrared intensity depends strongly on distance, so measure it with a ruler.
- Taking readings straight after filling. Wait so all faces reach the same temperature.
- Saying shiny surfaces absorb more. Shiny surfaces reflect infrared; matt black absorbs and emits the most.
Better results
- Use an infrared detector rather than a thermometer, as it responds faster and is more sensitive.
- Clamp the detector at a fixed distance instead of holding it.
- Repeat each reading and calculate a mean.
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7 exam questions on this practical.
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