Infrared radiation and emission/absorption

What you'll learn

This revision guide covers infrared radiation as a method of thermal energy transfer, focusing on how all objects emit and absorb infrared radiation depending on their temperature and surface properties. You'll learn how emission and absorption rates relate to surface characteristics, and how these principles apply to everyday situations and technological applications that appear in AQA GCSE Physics exams.

Key terms and definitions

Infrared radiation — electromagnetic waves with wavelengths longer than visible light that transfer thermal energy from one place to another without requiring particles or a medium.

Emission — the process by which an object gives out infrared radiation; all objects emit infrared radiation, with hotter objects emitting more radiation per second.

Absorption — the process by which an object takes in infrared radiation, causing its temperature to increase.

Black body — a theoretical perfect emitter and absorber of all wavelengths of radiation; real objects approximate black body behaviour to varying degrees.

Perfect absorber — an object that absorbs all radiation incident upon it, reflecting none; a black body is a perfect absorber.

Emissivity — a measure of how effectively a surface emits infrared radiation compared to a perfect black body at the same temperature (values between 0 and 1).

Surface temperature — the temperature of an object's outer surface, which determines the intensity and wavelength distribution of emitted infrared radiation.

Core concepts

All objects emit and absorb infrared radiation

Every object, regardless of its temperature, continuously emits infrared radiation. The rate at which an object emits this radiation depends on:

• Temperature: Hotter objects emit infrared radiation at a faster rate than cooler objects
• Surface area: Larger surface areas emit more total radiation
• Surface properties: The nature of the surface (colour, texture, material) affects emission rate

Simultaneously, all objects also absorb infrared radiation from their surroundings. Whether an object's temperature increases, decreases, or stays constant depends on the balance between emission and absorption:

• If absorption rate > emission rate → object heats up
• If emission rate > absorption rate → object cools down
• If absorption rate = emission rate → object maintains constant temperature (thermal equilibrium)

The intensity of infrared radiation from an object is proportional to its temperature. As temperature increases, objects emit:

• More infrared radiation per second (higher intensity)
• Radiation with shorter average wavelengths (shifts toward visible light at very high temperatures)

Surface properties and radiation

The colour and texture of a surface significantly affect how well it emits and absorbs infrared radiation. This is crucial for GCSE exam questions.

Dark, matt surfaces:

• Are good absorbers of infrared radiation
• Are good emitters of infrared radiation
• Heat up quickly when exposed to radiation
• Cool down quickly by emitting radiation
• Examples: matt black paint, charcoal, dark clothing

Light, shiny surfaces:

• Are poor absorbers of infrared radiation (good reflectors)
• Are poor emitters of infrared radiation
• Heat up slowly when exposed to radiation
• Cool down slowly by emitting radiation
• Examples: polished metal, white paint, aluminium foil, shiny kettle surfaces

Key principle: Good absorbers are also good emitters, and poor absorbers are also poor emitters. This is known as Kirchhoff's law of thermal radiation, though you don't need to name it at GCSE.

The Leslie cube experiment

The Leslie cube is a standard experimental demonstration used to investigate how surface properties affect infrared emission. You may be asked to describe this experiment or analyse results from it.

Apparatus:

• Metal cube with four different surface finishes (typically: matt black, matt white, shiny black, shiny silver)
• Hot water to fill the cube
• Infrared detector (thermopile or infrared thermometer)
• Ruler or fixed stand to maintain constant distance

Method:

1. Fill the cube with hot water to ensure all faces reach the same temperature
2. Place the infrared detector at a fixed distance from one face
3. Record the infrared radiation intensity detected
4. Rotate the cube and repeat for each face
5. Keep the detector at the same distance for all measurements

Results:

• Matt black surface emits most infrared radiation (highest detector reading)
• Shiny silver surface emits least infrared radiation (lowest detector reading)
• Matt white and shiny black surfaces show intermediate values

Conclusion: Dark, matt surfaces are better emitters of infrared radiation than light, shiny surfaces when at the same temperature.

Black body radiation

A black body is a theoretical ideal object that:

• Absorbs all radiation incident upon it (reflects none)
• Emits the maximum possible radiation at every wavelength for its temperature
• Has an emissivity of 1.0

While perfect black bodies don't exist in reality, some objects approximate this behaviour closely (such as cavities with small openings, or surfaces coated with carbon black).

Black body radiation characteristics:

• The intensity and wavelength distribution depends only on temperature
• As temperature increases, the peak wavelength shifts to shorter values
• Total energy emitted per second increases rapidly with temperature (proportional to T⁴ in the Stefan-Boltzmann law, though this equation is not required at GCSE)

Understanding black body radiation helps explain:

• Why hot objects glow (emission shifts into visible range)
• The colour of stars (hotter stars appear blue-white, cooler stars appear red)
• Design of thermal imaging cameras
• Energy efficiency in heating systems

Practical applications of emission and absorption

Solar water heating panels:

• Use matt black surfaces to maximize absorption of solar radiation
• Water pipes painted black absorb maximum infrared and visible radiation from the Sun
• Glass cover creates greenhouse effect, trapping heat

Survival blankets:

• Shiny metallic surface reflects infrared radiation back to the body
• Prevents heat loss through radiation in emergency situations
• Commonly used by marathon runners and mountain rescue teams

Radiators and cooling fins:

• Often painted with dark, matt paint to increase emission rate
• Increases efficiency of heat transfer to surroundings
• Computer heat sinks often have dark-coated fins

Teapots and kettles:

• Shiny surfaces reduce heat loss through radiation
• Keeps contents hot for longer periods
• Polished steel or chrome finishes commonly used

Clothing choices:

• Light-coloured, loose clothing in hot climates reflects radiation and reduces absorption
• Dark clothing in cold climates absorbs more solar radiation for warmth

Spacecraft thermal control:

• White or reflective surfaces on parts requiring cooling
• Black surfaces on radiator panels to maximize heat emission into space
• Critical for maintaining equipment temperatures in the vacuum of space

Comparing conduction, convection and radiation

Infrared radiation is one of three methods of thermal energy transfer. Understanding the differences is essential for exam questions:

Radiation:

• Does not require particles or a medium
• Can travel through a vacuum (e.g., Sun's energy reaching Earth)
• Travels at the speed of light
• Affected by surface properties (colour, texture)
• All objects emit and absorb radiation

Conduction:

• Requires particles in direct contact
• Energy transferred through particle vibrations (and free electrons in metals)
• Cannot occur through a vacuum
• Best in solids, especially metals

Convection:

• Requires particles that can move freely
• Energy transferred through bulk movement of fluid
• Only occurs in liquids and gases
• Cannot occur through a vacuum or in solids

In many real situations, all three methods occur simultaneously. For example, a radiator transfers heat by all three methods: conduction through the metal, convection currents in the air, and infrared radiation emission.

Worked examples

Example 1: Explaining observation (3 marks)

Question: Two identical cans are filled with hot water at the same temperature. Can A has a shiny silver surface. Can B has a matt black surface. After 30 minutes, the water in can B is cooler than the water in can A. Explain why.

Answer:

• Matt black surfaces are better emitters of infrared radiation than shiny silver surfaces (1 mark)
• Can B emits infrared radiation at a faster rate than can A (1 mark)
• Can B therefore loses thermal energy faster and cools down more quickly (1 mark)

Examiner note: The question asks you to "explain why" so you must link cause and effect. Simply stating facts about black and shiny surfaces without connecting them to the cooling rates would not gain full marks.

Example 2: Experimental design (4 marks)

Question: A student wants to investigate how surface colour affects the absorption of infrared radiation. Describe a method the student could use.

Answer:

• Take two identical metal plates/cans, one painted matt black and one painted matt white (1 mark)
• Place an equal volume of water at the same (room) temperature in each container (1 mark)
• Place both containers at an equal distance from the same heat source/lamp (1 mark)
• Measure the temperature of the water in each container at regular time intervals, OR measure the temperature after a fixed time (1 mark)

Alternative acceptable points:

• Use a thermometer to measure temperature
• Record results in a table
• The container with black surface should show greater temperature increase

Examiner note: Notice that the method must ensure a fair test — identical containers, equal volumes, same starting temperature, same distance from heat source. These control variables are essential for full marks.

Example 3: Application question (3 marks)

Question: Houses in hot countries are often painted white. Use your knowledge of infrared radiation to explain why this helps keep the houses cool.

Answer:

• White surfaces are poor absorbers of infrared/thermal radiation (1 mark)
• White surfaces reflect most of the radiation from the Sun (1 mark)
• Less thermal energy is absorbed by the house, so it stays cooler (1 mark)

Examiner note: This is an application question testing whether you can apply your knowledge to an unfamiliar context. The key is recognizing that white surfaces both absorb less radiation from the Sun and reflect more of it.

Common mistakes and how to avoid them

• Confusing emission with absorption: Remember that good absorbers are also good emitters. Students often correctly state that matt black surfaces absorb radiation well but then incorrectly claim they don't emit radiation well. Both absorption and emission follow the same pattern with surface properties.

• Claiming radiation needs particles or a medium: Unlike conduction and convection, infrared radiation can travel through a vacuum. This is how the Sun's energy reaches Earth. Don't say radiation needs air or particles to travel.

• Using vague terms like "heat radiation" without precision: Use the specific term "infrared radiation" in exams. While "thermal radiation" is acceptable, "heat" is a process of energy transfer, not a type of radiation. "Heat rays" is not acceptable scientific terminology.

• Not identifying control variables in experiments: When describing Leslie cube experiments or similar investigations, you must state that all surfaces are at the same temperature and the detector is at the same distance from each surface. Missing these controls loses marks.

• Confusing temperature with thermal energy: Infrared radiation transfers energy, which may increase temperature, but temperature and thermal energy are different. A large object at low temperature may contain more thermal energy than a small object at high temperature.

• Reversing the properties of surfaces: A common error is stating that shiny surfaces are good absorbers or that white surfaces emit radiation well. Remember: dark and matt = good absorber and emitter; light and shiny = poor absorber and emitter.

Exam technique for "Infrared radiation and emission/absorption"

• "Describe" vs "Explain" command words: "Describe" requires you to state what happens (e.g., "the matt black surface emits more infrared radiation"). "Explain" requires you to give reasons why (e.g., "because matt black surfaces are better emitters of infrared radiation than shiny surfaces"). Make sure you match your answer style to the command word.

• Use comparative language in comparison questions: When comparing two surfaces, use words like "more," "less," "faster," "greater," "better" rather than absolute statements. For example: "Can A cools faster than Can B" is better than just "Can A cools quickly."

• Link surface properties to outcomes: In application questions, make explicit connections between the surface property and the observed effect. Don't just describe what happens—explain the physics using the correct terminology about absorption and emission.

• Draw on real-world contexts: AQA often asks questions set in practical contexts (clothing, buildings, cooking equipment, survival equipment). Practice applying your knowledge to these unfamiliar situations rather than just memorizing facts about Leslie cubes.

Quick revision summary

All objects emit and absorb infrared radiation continuously. The rate depends on temperature and surface properties. Dark, matt surfaces are good absorbers and emitters; light, shiny surfaces are poor absorbers and emitters. Infrared radiation transfers thermal energy without requiring particles and can travel through a vacuum. Objects hotter than their surroundings emit more radiation than they absorb and cool down. These principles apply to solar panels, survival blankets, thermal clothing, and buildings. Experimental investigations use apparatus like Leslie cubes with infrared detectors to compare emission rates from different surfaces at the same temperature.