What you'll learn
Every object emits and absorbs infrared radiation, and the hotter it is, the more it radiates — this idea explains the temperature of the Earth and the colour of stars. For AQA GCSE Physics (higher tier) you need to understand black body radiation, how the emission and absorption of radiation depend on temperature and surface, and how the balance between absorbing and emitting radiation controls an object's temperature. This guide covers infrared radiation, perfect black bodies, the factors affecting emission and absorption, and how this balance keeps the Earth's temperature roughly steady. By the end you should be able to explain what a black body is and how the emission and absorption of radiation determine an object's temperature.
Key terms and definitions
Infrared radiation — Electromagnetic radiation emitted by all objects because of their temperature.
Black body — An idealised object that absorbs all the radiation that hits it and is also the best possible emitter.
Perfect black body — An object that absorbs all radiation and reflects or transmits none.
Emission — The giving out of radiation by an object.
Absorption — The taking in of radiation by an object.
Intensity — The amount of radiation emitted or received per second over a given area.
Equilibrium — A state where the rate of absorption equals the rate of emission, so temperature is constant.
Electromagnetic spectrum — The full range of electromagnetic waves, from radio waves to gamma rays.
Core concepts
All objects emit radiation
Every object emits and absorbs infrared radiation. The amount and type of radiation an object emits depends on its temperature. As an object gets hotter, it emits more radiation every second, and it also emits radiation across a wider range of wavelengths, including shorter wavelengths. This is why a piece of metal glows red and then white as it gets hotter — at higher temperatures it emits visible light as well as infrared.
Black bodies
A black body is an idealised object that is a perfect absorber of radiation — it absorbs all the radiation that falls on it and reflects or transmits none. A perfect black body is also the best possible emitter of radiation. Real objects are not perfect black bodies, but many behave approximately like them. Stars, for example, behave close to black bodies, which is why the colour of a star tells us about its temperature — hotter stars emit more short-wavelength (blue) light, while cooler stars appear redder.
Emission and absorption depend on the surface
The rate at which an object emits or absorbs radiation depends on its surface as well as its temperature:
- Dark, matt (dull) surfaces are good absorbers and good emitters of radiation.
- Light, shiny surfaces are poor absorbers and poor emitters, and good reflectors.
This is why, for example, a shiny surface stays cooler in sunlight (it reflects rather than absorbs), and why radiators and cooling fins are often painted matt black to emit heat well.
Temperature and the balance of radiation
An object's temperature depends on the balance between the radiation it absorbs and the radiation it emits:
- If an object absorbs more radiation than it emits, its temperature rises.
- If it emits more than it absorbs, its temperature falls.
- If it absorbs and emits radiation at the same rate, its temperature stays constant — it is in equilibrium.
As an object gets hotter, it emits more radiation, which tends to bring the rates of absorption and emission back into balance at a steady temperature.
The temperature of the Earth
This balance explains the temperature of the Earth. The Earth absorbs radiation from the Sun and emits radiation back into space. When these are balanced, the Earth's average temperature stays roughly constant. If the Earth absorbed more than it emitted — for example, if more radiation were trapped — its temperature would rise. This is the basis of understanding how changes to the atmosphere can affect global temperature, because they change how much radiation is emitted back into space.
Black body radiation and wavelength
A hotter object does not just emit more radiation overall; it also emits a greater proportion of shorter-wavelength radiation. At room temperature, objects emit mainly infrared, which we cannot see. As objects get much hotter, they emit visible light, moving from red heat to white heat. This link between temperature and the wavelengths emitted is what allows astronomers to work out the temperature of stars from their colour.
The greenhouse effect and global temperature
The balance of radiation that controls the Earth's temperature is affected by gases in the atmosphere. Radiation from the Sun, much of it short-wavelength, passes through the atmosphere and is absorbed by the Earth's surface, warming it. The Earth then emits longer-wavelength infrared radiation back towards space. Greenhouse gases, such as carbon dioxide and methane, absorb some of this outgoing infrared radiation and re-emit it, so less escapes to space. This keeps the Earth warmer than it would otherwise be. If the amount of greenhouse gas increases, less radiation escapes, so the Earth absorbs more than it emits and its average temperature rises. This connects black body ideas directly to climate change, a common exam link.
Using radiation to measure temperature
Because the radiation an object emits depends on its temperature, we can measure temperature by detecting radiation without touching the object. Thermal imaging cameras detect the infrared radiation emitted by objects and show hotter areas as brighter or differently coloured, which is used to find heat loss from buildings or to detect people in the dark. Astronomers use the same principle in reverse: by measuring the radiation and colour of a star, they work out its temperature, since hotter stars emit more radiation and more short-wavelength (bluer) light. These applications show why understanding the link between temperature and emitted radiation is so useful.
Worked examples
Example 1: The best absorber and emitter
What is a perfect black body? A perfect black body is an idealised object that absorbs all the radiation that falls on it, reflecting and transmitting none. It is also the best possible emitter of radiation at a given temperature.
Example 2: Choosing a surface
Why is a radiator sometimes painted matt black? A matt black surface is a good emitter of infrared radiation. Painting a radiator matt black means it emits heat to the surroundings more effectively than a shiny surface would, helping it warm the room.
Example 3: Explaining a rising temperature
An object absorbs radiation faster than it emits it. Explain what happens to its temperature. Its temperature rises. Because it is taking in more energy by radiation than it is giving out, the extra energy increases its internal energy, so its temperature increases until emission rises to balance absorption.
Example 4: The Earth's temperature
Explain how the Earth's temperature stays roughly constant. The Earth absorbs radiation from the Sun and emits radiation into space. When the rate of absorption equals the rate of emission, the Earth is in balance and its average temperature stays roughly constant. If this balance is disturbed, the temperature changes.
Common mistakes and how to avoid them
A common error is thinking only hot objects emit radiation. All objects emit infrared radiation because of their temperature; hotter objects simply emit more and at shorter wavelengths.
Students often mix up which surfaces are good emitters. Dark, matt surfaces are good absorbers and good emitters; shiny, light surfaces are poor at both and good reflectors. A good absorber is also a good emitter.
Another mistake is forgetting that temperature depends on the balance of absorption and emission. It is not about absorption alone — an object heats up only if it absorbs faster than it emits.
When explaining the Earth's temperature, do not just say it absorbs radiation. It both absorbs (from the Sun) and emits (to space), and the balance between the two determines the temperature.
Finally, remember a perfect black body is both the best absorber and the best emitter — students sometimes think a black body only absorbs.
Exam technique for "Black body radiation and emission of radiation"
Be ready to define a perfect black body as the best absorber and best emitter, and to explain that all objects emit and absorb infrared radiation depending on their temperature and surface.
For temperature questions, always explain in terms of the balance between absorption and emission: rising if it absorbs more, falling if it emits more, constant if they are equal. Apply this to the Earth's temperature, mentioning radiation from the Sun and back into space.
Link surface type to emission and absorption (matt black good, shiny poor), and explain that hotter objects emit more radiation and more short-wavelength radiation. Use precise terms — black body, emission, absorption, equilibrium — throughout.
Quick revision summary
- All objects emit and absorb infrared radiation; hotter objects emit more and at shorter wavelengths.
- A perfect black body absorbs all radiation and is also the best emitter.
- Dark, matt surfaces are good absorbers and emitters; shiny, light surfaces are poor at both and good reflectors.
- An object's temperature depends on the balance of absorption and emission: absorb more → warms; emit more → cools; equal → constant.
- The Earth's temperature stays roughly constant when radiation absorbed from the Sun equals radiation emitted to space.
- The colour of a hot object or star indicates its temperature, because hotter bodies emit more short-wavelength radiation.