6.6.2.2 Infrared Absorption and Radiation: Required Practical — Refraction and Wave Properties — AQA Combined Science Trilogy 8464

(HT only) Explain that various substances may absorb, transmit, refract, or reflect electromagnetic waves distinctly depending directly on the applied wavelength.
(HT only) Understand that physical effects, such as refraction, intrinsically occur due to the shifting velocity of waves as they transverse through differing substances.
Construct accurate ray diagrams vividly illustrating the refraction of a wave positioned at the direct boundary between two different media.
(HT only) Use comprehensive wave front diagrams to effectively explain refraction in terms of wave speed changes occurring during medium-to-medium travel.
Describe and perform a practical investigation studying how the amount of infrared radiation absorbed or radiated by a surface is heavily influenced by the nature of that specific surface.

How surfaces transfer energy by infrared radiation

A warm object transfers energy to its surroundings by emitting infrared radiation. Infrared is an electromagnetic wave: it carries energy without needing a material medium. You cannot see it, but an infrared detector can respond to it.

A surface can also absorb infrared arriving from elsewhere. Absorption transfers energy to the object and can increase its temperature. Emission means giving radiation out; absorption means taking it in.

The nature of a surface affects both processes. A matt black surface is a good absorber and emitter of infrared. A shiny silver surface is a poor absorber and emitter, and reflects much of the infrared that reaches it. ‘Matt’ means dull rather than shiny, so the finish matters as well as the colour.

Comparing emission with a Leslie cube

Required practical activity 21 investigates how infrared absorption or emission depends on the nature of a surface. The Leslie cube method investigates emission directly.

A Leslie cube is a hollow metal container whose four sides have different surfaces: shiny black, matt black, shiny silver and matt white. Filling it with hot water heats all four sides together. This lets you compare different surfaces at approximately the same temperature, rather than comparing separate objects that might be at different temperatures.

The apparatus is a Leslie cube, a kettle, an infrared detector and a heatproof mat. A ruler is useful for checking the distance between the detector and each face.

Cutaway schematic of a lidded Leslie cube containing very hot water and resting on a heatproof mat. One infrared detector faces the centre of a vertical side across an air gap. The distance between that face and the detector must stay the same when this single detector is moved to measure each face in turn. The cube's four vertical faces have shiny black, matt black, shiny silver and matt white finishes. — Compare radiation from each face using the same detector at the same distance.

Taking the measurements

Place the Leslie cube on the heatproof mat.
Use the kettle to heat water. Carefully fill the cube with very hot water and fit its lid.
Position the infrared detector facing the centre of one surface, without touching it. Measure and record its distance from that surface.
Allow the detector reading to settle, then record the surface type and its reading.
Measure each of the other three surfaces, keeping the detector at the same distance and facing each surface in the same way.
Repeat the measurements so that you can check consistency and calculate a mean reading for each surface.

Record readings with the units shown by the instrument. The readings allow you to compare the infrared detected from each surface; a larger reading indicates more infrared reaching the detector under the same measurement conditions.

Very hot water and the heated cube can cause burns or scalds. Keep the apparatus stable, handle the hot water carefully and keep water away from electrical equipment.

Making the comparison fair

The independent variable is the nature of the surface. The dependent variable is the infrared detector reading.

Distance must stay constant: moving the detector further away can reduce the radiation it receives. Keep the detector's position and angle consistent, and use the same detector throughout. The cube's faces have the same area, so surface area is not deliberately changed either.

Temperature is particularly important because hotter objects emit more infrared. The shared hot water helps keep the faces at approximately the same temperature, but the cube gradually cools. Take the readings promptly. Changing the order of the surfaces between repeated sets helps reveal whether cooling is affecting the comparison: a surface should not always be measured last.

Presenting results and drawing a conclusion

Use a results table with a row for each surface and columns for repeated detector readings and their mean. A bar chart is appropriate because the surface types are separate categories. Put surface type on the horizontal axis and mean detector reading, with its units, on the vertical axis. Do not join the bars with a line as though surface type were a continuous numerical variable.

Compare the heights of the bars. Under the same conditions, a higher bar means that the surface emitted more infrared towards the detector. Matt black is expected to be a strong emitter and shiny silver a weak emitter. Compare shiny black and matt white using your actual readings rather than assuming an exact order.

Your conclusion should connect the evidence to the aim: surfaces at approximately the same temperature can give different detector readings, showing that infrared emission depends on surface nature. This experiment measures emission, not absorption. The related pattern is that good infrared emitters are also good absorbers, but demonstrating absorption directly would require measuring the response to incoming radiation.

Surface behaviour

Matt black: good infrared emitter and absorber.
Shiny silver: poor emitter and absorber; good reflector.
Emission transfers energy away from an object; absorption transfers energy to it.

Leslie cube method

Place the cube on a heatproof mat, fill it with very hot water and fit the lid. Compare detector readings from shiny black, matt black, shiny silver and matt white faces.

Change: surface type.
Measure: infrared detector reading.
Control: detector distance and angle, face area and temperature as far as possible.
Measure promptly, repeat and compare mean readings. Vary the measurement order to check the effect of cooling.

Results

Use a bar chart: surface type against mean detector reading, including units. A higher reading under the same conditions indicates greater emission towards the detector.

The Leslie cube method tests emission, not absorption.

For Combined Science: Trilogy, this is required practical activity 21. Describe the apparatus and measurements, rather than relying on the practical number.

Name the independent variable as the nature of the surface: both colour and finish matter.

Explain controls: keeping the detector at the same distance prevents distance, rather than surface type, from causing a difference in readings.

A Leslie cube investigation measures emission. Do not describe its detector readings as measurements of absorption.

Electromagnetic radiation that transfers energy and is emitted by warm objects; it is invisible to the human eye.

The giving out of radiation by an object, transferring energy away from it.

The taking in of radiation by a material, transferring energy to it.

A hollow metal container with different surface finishes on its four sides, used to compare infrared emission when filled with hot water.

A dull, non-shiny surface finish.

An instrument that responds to infrared radiation, allowing radiation from different surfaces to be compared.