Absorbing radiation
When an object absorbs radiation, it heats up and its temperature will rise.
You feel the heat of the sun when you absorb the infrared radiation from it.
If a cloud passes in front of the sun, you absorb less infrared, and it feels cooler.
How infrared radiation is absorbed can be investigated using the apparatus shown below.
Two squares of aluminium are arranged as shown above.
One is painted dull black, the other is polished and shiny.
To ensure a fair test they are the same area and thickness and are placed the same distance from the Bunsen flame.
Two identical corks are stuck to the back of the plates using equal amounts of Vaseline or candle wax (control variables).
The Bunsen is lit.
Quite quickly the cork attached to the black plate falls off.
The cork behind the polished plate takes much longer to fall off.
Conclusion
Both plates receive the same quantity of radiation, but the black plate heated up more quickly.
This tells us that a dull, black surface is a better absorber of radiation than a shiny, polished surface.
Key points
- Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.
- Dark matt surfaces are better at radiating heat energy than light shiny surfaces.
| Surface | Absorption | Emission |
| Dull, matt or rough | Good absorber of heat radiation | Good emitter of heat radiation |
| Shiny | Poor absorber of heat radiation | Poor emitter of heat radiation |
| Surface | Dull, matt or rough |
|---|---|
| Absorption | Good absorber of heat radiation |
| Emission | Good emitter of heat radiation |
| Surface | Shiny |
|---|---|
| Absorption | Poor absorber of heat radiation |
| Emission | Poor emitter of heat radiation |
Applications
Teapots and saucepans are shiny so that they don’t emit too much radiation and cool down too quickly.
Cricketers wear white to remain cool in summer as white it is a good reflector (poor absorber) of radiation.
Shiny silver blankets are used to stop heat loss from a person pulled from a cold sea or evacuated from a swimming pool.
Solar Panels should be black because black is a good absorber of radiation.
Radiators are often painted with white gloss paint, but they would be better at radiating heat if they were painted with matt black paint.
They are painted white to make them look nicer.
However, despite their name, radiators actually transfer most of their heat to a room by convection not radiation.
So, radiators are the wrong colour, and have the wrong name.
Normally heaters need to be red hot before they are any good at heating by radiation.
