All objects above absolute zero emit infrared radiation. The hotter the object, the more infrared it emits, and the shorter the peak wavelength. A good absorber is also a good emitter — dull, dark surfaces absorb and emit infrared more effectively than shiny, light-coloured surfaces.
What is infrared radiation?
Infrared radiation (IR) is part of the electromagnetic spectrum, with wavelengths between approximately 700 nm and 1 mm — just beyond visible red light. Like all electromagnetic waves, infrared travels at the speed of light in a vacuum (3 × 10⁸ m/s) and requires no medium; it is the only form of heat transfer that works across the vacuum of space, which is why the Sun warms the Earth.
Infrared is often called thermal radiation at GCSE because its primary effect is to transfer thermal energy between objects. Any object above absolute zero (0 K, −273 °C) emits infrared radiation. The hotter an object, the more infrared it emits per second and the shorter the wavelength of the peak emission.
How does surface type affect the emission of infrared?
Not all surfaces emit infrared equally well at the same temperature. GCSE physics distinguishes two extremes:
Dull, dark (matt) surfaces:
- Emit infrared radiation strongly — they are good emitters
- Also absorb infrared radiation strongly — they are good absorbers
Shiny, light-coloured surfaces:
- Emit infrared radiation poorly — they are poor emitters
- Also absorb infrared radiation poorly — they are poor absorbers (good reflectors)
This symmetry — good absorbers are also good emitters — is a key principle. A surface that is efficient at taking in infrared is equally efficient at radiating it. A perfect absorber/emitter is called a black body (a theoretical ideal; in practice, matt black surfaces are a reasonable approximation).
What does the Leslie cube experiment show?
The Leslie cube is a hollow metal cube filled with hot water. Each face is coated with a different surface finish — typically matt black, matt white, shiny metal, and dull metal. A thermal camera or infrared detector is placed at the same distance from each face.
Results:
| Surface | Infrared emitted (relative) |
|---|---|
| Matt black | Most |
| Matt white | Nearly as much as matt black |
| Dull/dark metal | Moderate |
| Shiny (polished) metal | Least |
The experiment confirms that colour and surface texture, not just temperature, determine how much infrared is emitted. A matt black surface at 60 °C emits significantly more infrared than a shiny metallic surface at 60 °C.
Note: white surfaces are poor absorbers of visible light, but at the infrared wavelengths involved in thermal radiation, white paint is nearly as good a radiator as black paint — the visual appearance of a surface does not perfectly predict its infrared properties.
How does temperature affect infrared emission?
The rate of emission of infrared radiation increases sharply with temperature:
- A hotter object radiates more infrared energy per second from the same area
- A hotter object also emits at shorter peak wavelengths — very hot objects (e.g. the Sun at ~5,500 °C) emit visible light as well as infrared; cooler objects (e.g. the human body at 37 °C) emit only infrared
This is why thermal cameras can detect people in the dark: the human body continuously emits infrared at wavelengths around 9–10 µm, well beyond the range of the human eye but detectable by IR sensors.
What are the practical applications of infrared radiation?
| Application | How infrared is used |
|---|---|
| Thermal imaging cameras | Detect IR emitted by warm objects; used in medicine, search and rescue |
| Night vision equipment | Detects IR rather than visible light |
| TV remote controls | Transmit coded IR pulses to control devices |
| Grills and toasters | Heating elements emit IR to cook food |
| Greenhouse design | Dark-coloured central heating radiators emit more IR |
| Building insulation | Shiny foil reflects IR back into the house, reducing heat loss |
How does infrared radiation relate to conduction and convection?
There are three mechanisms of heat transfer: conduction, convection, and radiation. Infrared radiation is distinct because:
- Conduction requires physical contact and a solid medium — particles vibrate and pass energy along.
- Convection requires a fluid (liquid or gas) — warmer fluid rises, cooler fluid sinks.
- Radiation requires no medium — it can travel through a vacuum.
In a house, all three operate simultaneously: the roof loses heat by conduction through its tiles, convection in the loft air, and infrared radiation from its outer surface to the night sky. Reducing the emissivity of the outer surface (e.g. using reflective roofing materials) reduces radiation losses.
Why do solar panels and solar water heaters use dark surfaces?
Solar collectors need to absorb as much infrared (and visible) radiation from the Sun as possible. A dark, matt surface is the most efficient absorber — so solar thermal panels are coated in a dark material to maximise the energy they capture. Conversely, reflective mylar blankets are used by athletes or emergency workers to retain body heat — the shiny surface reflects infrared back towards the person rather than absorbing it.
Frequently asked questions
Why does a shiny metal surface feel colder to the touch than a matt black surface at the same temperature?
The surfaces are at the same temperature, so there is no difference in the amount of thermal energy they contain. The shiny surface feels cooler because it is a poor emitter of infrared — your hand absorbs less radiation from it. However, if you grip either surface, conduction will transfer heat at the same rate (assuming the same thermal conductivity). This demonstrates the difference between radiated and conducted heat transfer.
Does the colour of a surface matter for infrared absorption or only for visible light?
For infrared at thermal wavelengths (roughly 3–100 µm), what matters is surface finish (texture and emissivity), not visual colour. White and black paint have very similar emissivities in the thermal infrared range — both are much better radiators than polished metal. The visual colour of a surface predicts how well it absorbs visible light (short wavelengths), but you need to consider thermal emissivity to predict infrared behaviour.
How does infrared radiation heat a greenhouse?
Sunlight (including visible light and near-infrared) passes through glass and is absorbed by objects inside the greenhouse, warming them. These warm objects then re-emit long-wave infrared (thermal IR) — but glass is largely opaque to long-wave IR and so reflects or absorbs it rather than transmitting it back out. Energy is therefore trapped inside. This is the greenhouse effect. A similar process operates at planetary scale: the atmosphere is transparent to short-wave solar radiation but partly opaque (due to greenhouse gases) to the long-wave IR re-emitted from Earth's surface.
What is the link between infrared emission and the concept of a black body?
A black body is a theoretical perfect emitter and perfect absorber of radiation at every wavelength. Its emission spectrum (power output vs wavelength) depends only on temperature and follows a well-defined curve (the Planck curve). Real surfaces have emissivities between 0 (perfect mirror) and 1 (perfect black body). Matt black surfaces have emissivities close to 1; polished metals have emissivities close to 0.05. At GCSE, you only need the qualitative principles — more detail is studied at A-level.
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