Required Practical: Radiation and Absorption

This lesson covers the Required Practical on investigating how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface, as required by the AQA GCSE Physics specification (4.6.2). This practical investigates the link between surface colour/texture and the emission and absorption of infrared radiation.

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Overview

AQA requires you to investigate the emission and absorption of infrared radiation by different surfaces. The key finding is:

• Dark, matt (rough) surfaces are good absorbers and good emitters of infrared radiation.
• Light, shiny (smooth) surfaces are poor absorbers and poor emitters of infrared radiation (they are good reflectors instead).

This practical can be examined in multiple ways: you may be asked to describe the method for emission, absorption, or both. You must also be able to identify variables, explain sources of error, and interpret results.

Exam Tip: This is a commonly examined required practical. Remember the key conclusion: dark matt surfaces are better emitters and absorbers of infrared radiation than light shiny surfaces. You should be able to describe both the emission and absorption experiments clearly.

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Experiment 1: Investigating Infrared Emission

Equipment

Equipment	Purpose
Leslie cube (metal cube with four different surfaces)	Provides surfaces of different colours and textures at the same temperature
Boiling water	Fills the Leslie cube to provide a source of infrared radiation
Infrared detector (or thermometer)	Measures the intensity of infrared radiation emitted by each surface
Ruler	Ensures the detector is the same distance from each surface
Kettle	To boil the water
Heatproof mat	To protect the bench

Description of a Leslie Cube

A Leslie cube is a hollow metal cube with four different outer surfaces:

Surface	Description
Side 1	Matt black (dark and rough)
Side 2	Matt white (light and rough)
Side 3	Shiny silver (light and smooth/polished)
Side 4	Shiny black (dark and smooth/polished)

All four surfaces are at the same temperature because they are all sides of the same metal cube filled with hot water. This is a crucial control variable.

Method

1. Fill the Leslie cube with boiling water and allow it to stand for two minutes so that all four surfaces reach the same temperature.
2. Place the infrared detector at a fixed distance (e.g. 10 cm) from one surface of the cube.
3. Record the infrared reading (or temperature detected) from the detector.
4. Without moving the cube, rotate it so the detector faces a different surface. Keep the same distance from the detector.
5. Record the reading for each of the four surfaces.
6. Repeat the entire experiment at least three times and calculate a mean for each surface.

Expected Results

Surface	Infrared Emission (relative reading)	Emission Rating
Matt black	Highest	Best emitter
Matt white	Moderate	Moderate emitter
Shiny black	Low-moderate	Moderate-poor emitter
Shiny silver	Lowest	Poorest emitter

Conclusion

• Dark, matt surfaces emit more infrared radiation than light, shiny surfaces.
• The matt black surface emits the most; the shiny silver surface emits the least.
• Colour and texture both affect emission, but matt/shiny (texture) has a larger effect than colour alone.

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Experiment 2: Investigating Infrared Absorption

Equipment

Equipment	Purpose
Two metal plates (or cans) — one matt black, one shiny silver	To compare absorption rates
Radiant heater (infrared lamp)	Source of infrared radiation
Thermometers (or temperature sensors)	Measure the temperature rise of each plate
Stopwatch	Times the experiment
Ruler	Ensures both plates are equidistant from the heater
Clamp stands	Hold the plates in position

Method

1. Set up two identical metal plates (or cans of water) — one painted matt black and the other left shiny silver.
2. Place a thermometer on the back of each plate (or inside each can).
3. Position both plates at the same distance from a radiant heater (infrared lamp).
4. Record the starting temperature of both plates.
5. Switch on the heater and record the temperature of each plate every 30 seconds for 10 minutes.
6. Plot a graph of temperature against time for both plates.
7. Repeat the experiment at least once more and calculate means.

Expected Results

Time (minutes)	Matt Black Temp Rise (degrees C)	Shiny Silver Temp Rise (degrees C)
0	20	20
2	28	22
4	35	24
6	41	26
8	46	27
10	50	28

graph TD
    subgraph "Temperature vs Time Graph"
        MB["Matt black: steep curve - large temperature rise"]
        SS["Shiny silver: gentle curve - small temperature rise"]
    end
    style MB fill:#2c3e50,color:#fff
    style SS fill:#bdc3c7,color:#333

Conclusion

• The matt black plate absorbs infrared radiation much more effectively than the shiny silver plate, as shown by the larger temperature rise.
• Dark, matt surfaces are better absorbers of infrared radiation.
• Light, shiny surfaces are poor absorbers — they reflect most of the infrared radiation instead.

Exam Tip: When interpreting results, always refer to the data. For example: "The matt black surface showed a temperature rise of 30 degrees C in 10 minutes, compared to only 8 degrees C for the shiny silver surface. This shows that dark, matt surfaces absorb infrared radiation more effectively than light, shiny surfaces."

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Variables

Variable Type	Emission Experiment	Absorption Experiment
Independent	Surface type (colour and texture)	Surface type (matt black vs shiny silver)
Dependent	Infrared detector reading	Temperature rise
Control	Distance from detector, temperature of water in cube, room temperature	Distance from heater, starting temperature, heater power, plate size and material, room temperature

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Safety Precautions

Hazard	Risk	Precaution
Boiling water	Scalding	Use a kettle with a lid; pour carefully; wear gloves or use tongs
Hot surfaces (Leslie cube, heater)	Burns	Do not touch the cube or heater during the experiment; use a heatproof mat
Radiant heater	Burns if touched or if skin is too close	Keep a safe distance; do not look directly at the heater element
Infrared lamp	Eye damage	Avoid looking directly at the lamp

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Sources of Error and How to Reduce Them

Source of Error	Effect	How to Reduce
Draughts in the room	Cooling by convection affects results unevenly	Close windows and doors; use a draught shield
Unequal distance from detector to surfaces	Some surfaces appear to emit more because the detector is closer	Use a ruler to ensure the same distance each time
Water cooling during measurements	Temperature of the Leslie cube drops over time	Take readings quickly; refill with hot water if needed
Background infrared from other sources	Affects detector readings	Take a background reading with no heat source and subtract it
Starting temperatures not equal (absorption)	Unfair comparison	Ensure both plates start at the same temperature
Thermometer reading errors	Inaccurate temperature values	Use digital thermometers for better precision; read at eye level

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Practical Applications

Understanding infrared absorption and emission has real-world applications: