Required Practical 6: Infrared Radiation and Surfaces

Section 1 of 9Key idea

Emitters

• Emitters All objects emit infrared radiation, and hotter objects emit more.
• Dark, matt surfaces are the best emitters; light, shiny surfaces are the poorest.

Absorbers

• Absorbers Dark, matt surfaces are also the best absorbers of infrared; shiny surfaces reflect it instead.

Section 2 of 9Leslie cube method

1

Fill a Leslie cube, which has four different faces (matt black, shiny black, matt white, shiny silver), with boiling water.

2

Place an infrared detector at the same distance, a few centimetres, from each face in turn.

3

Record the detector reading for each face.

4

Repeat the measurements and calculate a mean for each surface.

5

Compare the means to rank the surfaces.

Section 3 of 9Variables

Type	Variable
Independent	Type of surface (colour and finish)
Dependent	Amount of infrared detected
Control	Water temperature, detector distance, angle and detector type

Section 4 of 9Absorption version

1Identical flasks

• Identical flasks Use boiling tubes or flasks coated in different surfaces, each filled with the same volume of water.

2Equal heating

• Equal heating Place a heater equal distances from each and heat for the same time.

3Measure rise

• Measure rise Compare temperature rise; the biggest rise means the best absorber.

Section 5 of 9Safety and reliability

Safety

• Safety Boiling water causes scalds, so use a kettle carefully and keep the cube on a heat mat.

Reliability

• Reliability Keep the room free from draughts and other heat sources, and repeat readings.

Section 6 of 9Processing results

Surface	Typical detector reading	Conclusion
Matt black	Highest	Best emitter of infrared
Shiny black	High	Good emitter, but not as good as matt black
Matt white	Low	Poor emitter
Shiny silver	Lowest	Worst emitter of infrared

Section 7 of 9Applying the results

Radiation in everyday life

Radiators

• Radiators Painting a radiator matt black helps it emit more infrared into the room.

Survival blankets

• Survival blankets Shiny silver foil reflects infrared back towards the body and emits little, so it keeps people warm.

Keeping cool

• Keeping cool Light, shiny roofs and clothing reflect infrared from the Sun and absorb little of it.

Kettles and flasks

• Kettles and flasks Silvered surfaces on a vacuum flask reduce energy transfer by radiation.

Section 8 of 9Key terms

Term	Meaning
Infrared radiation	Electromagnetic waves emitted by all objects because of their temperature.
Emitter	A surface that gives off radiation.
Absorber	A surface that takes in radiation.
Matt	A dull, rough surface that scatters light and radiation.
Leslie cube	A metal cube with four differently coated faces, filled with hot water to compare infrared emission.
Infrared detector	A device that measures the amount of infrared radiation reaching it.

Section 9 of 9Exam technique

1

Thinking shiny surfaces are the best emitters. They are the worst.

2

Letting the detector distance vary between faces, which makes the test unfair.

3

Saying heat is the thing detected. The detector measures infrared radiation.

4

Forgetting to take the mean of repeated readings before comparing the surfaces.