Radiation and absorption required practical

Aim: To investigate how the amount of infrared radiation emitted or absorbed by a surface depends on its colour and texture, using a Leslie cube.

Variables

• Independent: Type of surface (e.g. matt black, shiny black, matt white, shiny silver)
• Dependent: Amount of infrared radiation emitted (reading on the infrared detector)
• Control: Temperature of the water inside the cube; Distance between the detector and each surface; Time between filling the cube and taking readings; Using the same detector

Equipment

• Leslie cube with four different faces
• Kettle for very hot water
• Infrared detector or thermometer
• Ruler
• Heatproof mat
• Optional: two metal plates, one matt black and one shiny, with wax and drawing pins for the absorption test

Method

1. Place the Leslie cube on a heatproof mat.
2. Fill it with very hot water from a kettle and put the lid on.
3. Wait a minute for the faces to heat up to the same temperature as the water.
4. Hold the infrared detector a fixed distance, for example 10 cm, from the first face, measuring with a ruler.
5. Record the reading, then turn the cube and take a reading from each of the other faces at the same distance.
6. Repeat the readings and calculate a mean for each surface.
7. Absorption (optional): fix a drawing pin with wax to the back of a matt black plate and a shiny plate, place both the same distance from a heater and time how long each pin takes to fall.

Safety

• Take great care pouring very hot water: use a jug with a spout and do not move the cube once it is full.
• Do not touch the Leslie cube or the metal plates while they are hot.
• Keep the cube on a heatproof mat on a stable bench away from the edge.

Results and calculations

Record the infrared reading for each surface in a table and draw a bar chart, since the surface type is a categoric variable. Matt black gives the highest reading (best emitter) and shiny silver the lowest; in the absorption test the pin on the matt black plate falls first, showing it is also the best absorber.

Common mistakes and improvements

• Holding the detector at different distances from each face: measure the distance with a ruler every time, or clamp the detector in place.
• The water cools while readings are taken, so later faces seem to emit less: take readings quickly and in a varied order, then repeat.
• The detector picks up radiation from other sources, such as your hand or a window: keep other warm objects away.
• Using a thermometer with poor resolution makes differences hard to see: use a digital infrared detector for better resolution.

Exam tips

• Remember the rule: matt black surfaces are the best emitters and absorbers; light, shiny surfaces are the worst emitters and absorbers but the best reflectors.
• Explain why a bar chart is used: the independent variable is categoric.
• Explain why all faces start at the same temperature: the cube holds one body of water, so the only variable changing is the surface.
• Name the key control variables clearly, especially distance from the surface and water temperature.
• Apply the idea to real contexts, such as why solar panels are black or why emergency blankets are shiny.