How does the nature of a surface affect the infrared radiation it emits and absorbs?
Infrared radiation and surfaces (core practical): how surface colour and texture affect the emission and absorption of infrared radiation, and the link to all objects emitting radiation.
A focused answer to Edexcel GCSE Physics 5.19 core practical (separate physics), covering how the nature of a surface affects the infrared radiation it emits and absorbs, that all objects emit and absorb infrared, the Leslie cube method, and the everyday consequences.
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What this dot point is asking
Edexcel core practical 5.19 (separate physics) wants you to investigate how the nature of a surface affects the amount of thermal (infrared) radiation it emits or absorbs, and to recall that all objects emit and absorb infrared radiation, with the amount depending on the surface and temperature.
All objects emit and absorb infrared
Infrared is the part of the spectrum just beyond red, and it is the radiation associated with the temperature of an object. The hotter something is, the more infrared energy it radiates each second, which is how a thermal imaging camera "sees" warm objects in the dark. An object at a steady temperature is emitting and absorbing infrared at equal rates.
How the surface affects emission and absorption
The colour and texture of a surface strongly affect how it exchanges infrared with its surroundings. A matt black surface absorbs almost all the radiation that lands on it and also radiates strongly when hot; a shiny silver surface reflects most incoming radiation and radiates weakly. This is why solar panels and the backs of radiators are often dark, while shiny foil is used to reflect heat.
The Leslie cube core practical
To make the test fair, the same hot water heats all the faces to the same temperature, and the detector is kept at a fixed distance and orientation. The only thing changed is the surface, so any difference in the detector reading is due to the surface. The independent variable is the surface, the dependent variable is the infrared detected, and the controls are the temperature and the detector distance.
How Edexcel examines this
This separate-physics core practical is examined on both tiers as a method question worth four to six marks, plus shorter application questions. The method question rewards describing the Leslie cube filled with hot water, an infrared detector held at a constant distance from each face, testing each surface, and identifying the independent, dependent and control variables; the conclusion that matt black emits the most infrared is often required. Application questions ask you to explain everyday situations, why a matt black object heats up or cools down faster than a shiny one, where the mark scheme rewards linking matt black to being a good absorber and emitter and shiny surfaces to being good reflectors and poor emitters. The single most penalised error is confusing good absorbers with good reflectors, so keep "matt black absorbs and emits well" firmly fixed. You may also be asked to connect this to energy transfer and cooling (a hot drink in a shiny vacuum flask stays warm because the shiny surfaces are poor emitters), linking the practical to the conservation-of-energy topic.
Try this
Q1. State which type of surface is the best emitter of infrared radiation. [1 mark]
- Cue. A matt black surface.
Q2. State the independent variable in the Leslie cube experiment. [1 mark]
- Cue. The type (nature) of the surface being tested.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20214 marksDescribe a core practical using a Leslie cube to investigate how the nature of a surface affects the amount of infrared radiation it emits. State the variables.Show worked answer →
Fill a Leslie cube (a metal cube with faces of different surfaces, for example matt black, shiny black, matt white and shiny silver) with hot water (1 mark). Use an infrared detector held the same distance from each face to measure the infrared emitted from each surface in turn (2 marks for the detector at a fixed distance and testing each face). The independent variable is the type of surface, the dependent variable is the infrared detected, and the control variables are the temperature of the water and the distance of the detector (1 mark). Markers reward the hot-water cube, an infrared detector at a constant distance, testing each surface, and identifying the variables. The matt black surface emits the most infrared.
Edexcel 20223 marksA car is left in the sun. Explain why a matt black car interior becomes hotter than a shiny silver one, using ideas about infrared radiation.Show worked answer →
A matt black surface is a good absorber of infrared (and other) radiation, so it absorbs more of the radiation from the Sun and warms up more (2 marks). A shiny silver surface is a poor absorber and a good reflector, so it absorbs less radiation and stays cooler (1 mark). Markers reward stating that matt black absorbs more radiation (good absorber) while shiny silver reflects more (poor absorber), leading to the temperature difference. Confusing good absorbers with good reflectors is the usual error.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)