6.10 Required practical: infrared radiation from surfaces

Aim

To investigate how the amount of infrared radiation emitted by a surface depends on the nature of the surface: for example, matt black, matt white, dull metal and shiny metal.

All objects emit infrared radiation. The hotter the object, the more it emits. The nature of the surface also matters: some surfaces are better emitters than others. The same surfaces are also better or worse absorbers.

Apparatus

• a Leslie cube with four faces (matt black, matt white, dull metal and shiny metal)
• a kettle to boil water, and a heat-resistant mat
• an infrared detector (or a radiation sensor and data logger)
• a ruler to measure the distance, a thermometer, and a stopwatch

Method

1. Place the Leslie cube on a heat-resistant mat. Carefully fill it with hot water from a kettle, to the same level each time, and put the lid on. Use a thermometer to check the temperature of the water.
2. Place the infrared detector a fixed distance (for example 5 cm) from the first face. The detector should be pointing at the middle of the face.
3. Read the detector and record the reading. Do this as soon as the reading is steady.
4. Turn the cube so that the next face is in front of the detector and measure again at the same distance. Do this quickly, since the water cools down.
5. Repeat for all four faces. Then repeat the whole set in a different order, and calculate a mean for each surface.

Variables

Results

The matt black surface gives the highest reading, and the shiny metal surface the lowest. Therefore:

• dark, matt surfaces are good emitters of infrared radiation;
• light, shiny surfaces are poor emitters.

The same is true for absorption: a matt black surface is also the best absorber of infrared radiation, while a shiny surface reflects most of it. You can check this by placing two metal plates, one matt black and one shiny, the same distance from a radiant heater, and measuring how quickly their temperatures rise: the matt black plate warms up faster.

Uses

These results explain real-life situations:

• Solar panels and the radiators of central heating are often painted matt black, to absorb or emit as much radiation as possible.
• Emergency blankets and the inside of a flask are shiny, to reflect radiation and reduce heat transfer.
• Houses in hot countries are often painted white, to reflect radiation and stay cooler.

Errors and precautions

• The water cools down during the experiment, so the measurements need to be done quickly and the order changed to remove bias. Alternatively, use a cube kept at a constant temperature.
• Keep the distance and the angle of the detector the same each time.
• The detector may also pick up infrared from other sources such as the room. Shield the detector, and record the reading with no cube to correct for it.
• Safety: boiling water can cause scalds. Fill the cube carefully on a mat, with the lid on, and do not move the cube while it is full.