Interactive practical

Infrared radiation and surfaces

Measure the infrared emitted by each face of a hot Leslie cube with a detector, then time pins dropping from matt black and shiny silver plates beside a heater.

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GCSE Physics

Infrared radiation and surface type

AQA 8465 — matched to your course.

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Practical focus

Compare how surface finish affects the emission or absorption of infrared radiation.

Practical method

Aim

Investigate how the type of surface affects the amount of infrared radiation emitted, using a Leslie cube.

Variables

Independent variable: type of surface (matt black, shiny black, matt white, shiny silver)
Dependent variable: infrared detector (or thermometer) reading
Control variables: distance from the detector to each face; water temperature in the cube; time each reading is taken.

Apparatus

Leslie cube with four different faces
Kettle of hot water
Infrared detector (or thermometer with a blackened bulb)
Ruler
Heatproof mat

Method

Place the Leslie cube on a heatproof mat and fill it with very hot water from a kettle.
Place the infrared detector 10 cm from the matt black face and record the reading.
Turn the cube and repeat for each face at the same distance, working quickly so the water cools very little.
Repeat the set of readings and calculate means.
Rank the surfaces from best to worst emitter.

Sample results

Leslie cube filled with water at about 80 °C; detector 10 cm from each face.

Surface	Detector reading (arbitrary units)
Matt black	9.2
Shiny black	8.0
Matt white	3.5
Shiny silver	1.4

Analysis

Matt black is the best emitter of infrared and shiny silver is the worst. Dark, dull surfaces emit (and absorb) infrared well; light, shiny surfaces emit poorly and reflect infrared. All faces are at the same temperature, so the differences are due only to the surface.

Errors and improvements

Problem	Effect	Improvement
Distance to each face not the same	Unfair comparison	Use a ruler or a fixed stand for the detector
Water cools between readings	Later readings lower	Work quickly; repeat in a different order and average
Infrared from other sources (hands, lamps)	Readings too high	Keep hands away and switch off heaters

Risk assessment

Hazard	Risk	Control measure
Very hot water	Scalds	Pour carefully on a heatproof mat; do not move the full cube
Hot metal cube	Burns	Do not touch the faces; let it cool before emptying

Common mistakes

Saying shiny surfaces are good emitters because they look bright.
Moving the detector closer to some faces.
Confusing emission with reflection.

Worked analysis

Mean readings for matt black: 9.1, 9.3 and 9.2. Mean = (9.1 + 9.3 + 9.2) ÷ 3 = 9.2. Matt black emits about 9.2 ÷ 1.4 ≈ 6.6 times as much as shiny silver.

Common misconception

Shiny surfaces are poor emitters and poor absorbers of infrared; they are good reflectors.

Variations required by some exam boards

Cooling curves: fill identical beakers wrapped in different surfaces with the same volume of hot water and record the temperature every minute. The matt black beaker cools fastest because it emits infrared best.

Absorption: place matt black and shiny silver plates the same distance from a heater. The matt black plate warms fastest, because good emitters are also good absorbers.

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