Required practical 21 · Physics · Physics Paper 2, both tiers
Infrared radiation from surfaces
Investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.
The exam trap. Surfaces absorb or emit infrared radiation, not temperature. Say which one your experiment measured.
Variables
- Independent (you change)
- The type of surface (matt black, shiny black, matt white, shiny silver)
- Dependent (you measure)
- Infrared radiation emitted, measured by a detector reading
- Control (you keep the same)
- Distance from detector to surface
- Temperature of the water in the cube
- Time the reading is taken
Apparatus
- Leslie cube with four different faces
- Kettle for hot water
- Infrared detector (thermopile) or thermometer with a blackened bulb
- Heatproof mat
- Ruler
Method, and why each step matters
Place the Leslie cube on a heatproof mat and fill it with very hot water.
Why Each face then starts at the same temperature.
Hold the infrared detector 10 cm from one face and record the reading.
Why The distance must be the same for each face for a fair comparison.
Turn the cube and record the reading for each face in turn, quickly.
Why Working quickly means the water cools very little between readings.
Repeat the set of readings and calculate a mean for each face.
Why Repeats reduce random error.
Compare the readings to rank the surfaces as emitters.
Why The highest reading shows the best emitter.
For absorption, place two plates (matt black and shiny) the same distance from a heater and compare their temperature rise.
Why Good emitters are also good absorbers.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Hot water can scald. | Fill the cube on the mat, do not move it when full, and let it cool before emptying. |
A worked set of results
| Surface | Detector reading (mean) |
|---|---|
| matt black | 85 |
| shiny black | 70 |
| matt white | 60 |
| shiny silver | 25 |
- Matt black surfaces emit the most infrared; shiny, light surfaces the least.
- The same surfaces that emit well also absorb well.
Mistakes that cost marks
Saying the surface absorbs temperature.
Instead Surfaces absorb or emit infrared radiation; temperature is a result.
Holding the detector at different distances from different faces.
Instead Keep the distance the same for every face.
Mixing up absorption and emission in the conclusion.
Instead Say which one the experiment measured: the Leslie cube measures emission.