Infrared radiation – which surfaces emit and absorb best
Integrated ScienceEnergyAges 13–14
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Sign in to playExperiment A: a Leslie cube full of hot water has matt black, matt white, dull silver and shiny silver faces. Turn each face towards an infrared detector at a fixed distance, zero the detector, record the readings in a table and compare them on a bar chart while the water slowly cools. Experiment B: a matt black plate and a shiny silver plate stand the same distance from a heater, each with a drawing pin stuck to the back with wax; time how long each pin takes to drop and watch the temperature graph of both plates. A fair-test checklist, an automatic conclusion and a note explain why white paint emits infrared almost as well as black paint.
Lesson: Thermal radiation: emission and absorption of infrared by different surfaces (Leslie cube)
What it shows
All objects emit infrared radiation, and hotter objects emit more. How much a surface emits at a given temperature depends on its emissivity: matt black surfaces are close to perfect emitters, while shiny metal emits very little because it reflects infrared. A good absorber is also a good emitter. In this model the detector reading is proportional to ε(T⁴ − T_room⁴) and falls with distance. White paint looks very different from black paint, but at the infrared wavelengths given off by warm water (about 10 µm) it behaves almost like a black surface.
How to use
In A. Emission – Leslie cube, choose a face (Matt black, Matt white, Dull silver, Shiny silver), press Zero detector, wait for a steady reading and press Record result. Auto-measure 4 faces does this for you. Use Refill hot water if the water has cooled. In B. Absorption – pins and wax, press Switch on heater and see which pin drops first; change Heater and Plate–heater distance, then press New run.
Parameters you can change
- Experiment A. Emission – Leslie cube, B. Absorption – pins and wax
- Face towards the detector Matt black, Matt white, Dull silver, Shiny silver
- Hot water temperature 40–90 °C
- Detector distance 5–30 cm
- Room temperature 15–30 °C
- Heater power 500–2000 W
- Plate–heater distance 4–15 cm
Questions to explore
- Why must the detector distance and the water temperature stay the same when you compare the faces?
- Why does the matt white face emit almost as much infrared as the matt black face?
- Why does the pin on the matt black plate drop before the pin on the shiny silver plate?
