Infrared Emission, Absorption and the Temperature of a Body
Ordinary objects above absolute zero emit thermal radiation. Emission depends on temperature and surface properties. Controlled comparisons of matt black paint and polished metal show how surfaces affect radiative heating and cooling. Earth's average temperature also depends on absorbed sunlight and thermal radiation escaping to space.
What a learner can do afterwards
- Predicts how matt black paint and polished metal affect radiative heating and cooling in a controlled comparison
- Uses total energy input and output rates to explain temperature changes for an object with fixed heat capacity and no change of state
- Explains how increased atmospheric infrared absorption can disturb Earth's energy balance and change average surface temperature
1 · Read
You have met infrared in the electromagnetic spectrum. Ordinary objects above absolute zero emit thermal radiation, mostly infrared at everyday temperatures. Radiation needs no material medium, so sunlight can cross space and warm skin. Conduction needs matter in contact; convection carries energy in moving fluids.
Matt black paint usually absorbs and emits infrared well. Polished metal usually reflects infrared and emits little at the same temperature. Good absorption and good emission are linked at the same wavelength. Visible colour alone does not tell you infrared behaviour: white paint can emit infrared well too.
Compare identical cans with equal heat capacities and initial temperatures above the same cooler, uniform surroundings. Match exposed areas and radiation exposure. With other transfers matched, matt black paint usually gives greater net radiative loss than polished metal. Without phase changes or changes in other energy stores, this means faster cooling. In sunlight, ordinary black paint absorbs more, but warming also depends on heat capacity and losses.
Only thermal energy stores change in this model. Mass and heat capacity stay fixed, and no phase change occurs. Net power is total input minus total output. Positive net power warms the object; negative net power cools it. Zero net power keeps its temperature steady while transfers continue. With equal heat capacities, larger positive net power gives faster warming. Count every transfer, including conduction, convection, radiation and internal heating.
Start with the Earth-atmosphere system in energy balance: absorbed sunlight equals radiation escaping to space. The surface emits infrared. Greenhouse gases absorb some infrared and emit upward and downward. Suppose increased absorption reduces the initial escape of energy while absorbed sunlight stays fixed. The system then gains energy and warms until outgoing radiation again matches absorbed sunlight.
For fixed surface area and unchanged emissivity, total emitted thermal power is proportional to T^4. Emissivity measures how effectively a surface emits compared with an ideal emitter. T is temperature in kelvin. Total emission includes all wavelengths, not just infrared. Net radiative loss also accounts for radiation absorbed from the surroundings.
Surface properties affect radiation. With fixed heat capacity, no phase change and no changes in other energy stores, balanced total input and output keep temperature steady.
This lesson, its questions and flashcards were written for this dataset. Scientific source records were used for facts and teaching design; their wording is not reproduced.
The readings and flashcards were written for this dataset. Blackbody radiation is a linked Khan Academy video. The video remains subject to publisher and YouTube terms; no reuse licence is asserted here.
More to read
Choosing the boundary of Earth's energy budget
For a planetary energy budget, place the boundary around both Earth and its atmosphere. Sunlight absorbed inside this boundary supplies energy. Reflected sunlight leaves without being absorbed. Thermal radiation escaping to space removes energy.
The surface emits infrared, but its upward emission is not all an immediate loss from the planetary system. Greenhouse gases absorb some infrared and emit radiation in all directions. Some atmospheric radiation travels downward and is absorbed by the surface. Energy may be exchanged several times before leaving for space.
Start from a balanced state. Suppose a change in greenhouse absorption reduces the outgoing energy rate while absorbed sunlight remains fixed. Input now exceeds output, so the system gains energy. Warming increases thermal emission until the outward rate again matches the absorbed sunlight.
This whole-system balance differs from the surface budget. The surface also exchanges energy with the atmosphere through processes such as convection and evaporation.
Visible appearance and infrared behaviour
Visible colour describes a surface's response to visible light. It does not give a complete description of that surface at infrared wavelengths. White paint can reflect visible light while emitting infrared strongly. Polished metal usually has much lower infrared emissivity than ordinary paint.
A useful cooling comparison changes the surface while keeping other conditions matched. Use identical cans, equal amounts of the same water and equal initial temperatures above the same cooler, uniform surroundings. Match exposed areas and radiation exposure. Account for other heat transfers. Matt black paint usually gives the greater net radiative loss. With equal heat capacities, no phase changes and no changes in other energy stores, that loss produces faster cooling.
Absorption and emission are linked at a given wavelength. That relation does not say that absorption of visible light must equal emission of infrared.
The fourth-power law concerns total emitted thermal power. Its temperature is in kelvin, and comparisons require fixed area and unchanged emissivity. To find net radiative loss, also count the radiation absorbed from the surroundings.
When an energy gain changes temperature
A temperature prediction needs an energy budget and a model of the object. Count all energy entering and leaving each second. Radiation may act alongside conduction, convection, electrical heating or evaporation.
In this model, only the thermal energy store changes. Mass and heat capacity stay fixed, and no phase change occurs. Positive net power increases thermal energy and raises temperature. Negative net power decreases thermal energy and lowers temperature. Equal total rates maintain a steady temperature even while energy continues to flow. Two objects with equal heat capacities warm at different rates if their positive net powers differ.
These conditions matter. Melting material can gain energy without a temperature rise. An object can also store energy in another form, so an energy gain alone does not always prove warming.
A powered heater gives another useful distinction. Its emitted radiation can exceed the radiation it absorbs while its temperature remains steady. Electrical input supplies the difference. Equal absorption and emission describes its steady state only if radiation is the sole transfer.
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