Thermal radiation

All objects emit thermal radiation in the form of infrared radiation. This radiation carries energy away from the object that emits it.

When another object absorbs the radiation, it gains thermal energy. Its temperature may rise as a result.

Thermal radiation does not require a material medium to travel through. It can therefore transfer energy across a vacuum, where there are no particles to carry the energy between the objects.

Surface properties and infrared radiation

When infrared radiation reaches a surface, some is absorbed and some is reflected. A surface also emits infrared radiation. Its colour and texture affect how well it does each of these.

Colour and infrared radiation

A black surface is a better absorber and emitter of infrared radiation than a white surface under the same conditions. A white surface is a better reflector of infrared radiation than a black surface.

Texture and infrared radiation

A dull surface is a better absorber and emitter of infrared radiation than a shiny surface under the same conditions. A shiny surface is a better reflector of infrared radiation than a dull surface.

These effects combine: a dull black surface is a good absorber and emitter of infrared radiation, whereas a shiny white surface is a good reflector and a poor absorber and emitter.

Energy balance and temperature

An object can receive energy while also transferring energy to its surroundings. Its temperature depends on the balance between the total rate of energy transfer into it and the total rate of energy transfer out of it.

• If the rates are equal, the object's temperature remains constant. Energy can still be entering and leaving; neither rate has to be zero.
• If energy enters faster than it leaves, the object's thermal energy increases and its temperature rises.
• If energy leaves faster than it enters, the object's thermal energy decreases and its temperature falls.

An object may transfer energy by more than one process at the same time, so all the rates of transfer in each direction must be included in the balance. For example, if an object receives energy at $$10,\text{W}$$ and transfers energy away at rates of $$6,\text{W}$$ and $$4,\text{W}$$ by two processes, the total rate out is $$10,\text{W}$$. The rates are equal, so its temperature remains constant.

Earth’s radiation balance

Earth receives energy from the Sun as radiation. Some of this incoming radiation is reflected back into space by the atmosphere, clouds and Earth’s surface. The remainder is absorbed by the Earth system, warming it.

Earth’s surface emits energy as infrared radiation. Some of this radiation is absorbed by the atmosphere, which then emits infrared radiation in different directions. Some is emitted towards the surface, while some travels towards space.

Radiation balance and temperature

Earth’s overall temperature depends on the balance between the rate at which it absorbs incoming energy and the rate at which it emits energy to space:

• If absorbed incoming energy exceeds energy emitted to space, Earth gains energy and warms.
• If energy emitted to space exceeds absorbed incoming energy, Earth loses energy and cools.
• If the two rates are equal, Earth’s overall temperature remains constant.

A change in the fraction of incoming radiation that is absorbed can therefore change Earth’s temperature. A change in the amount of infrared radiation escaping to space can also change the balance: for example, if less infrared energy escapes while the rate of absorption stays the same, Earth gains energy and warms.

Temperature and area effects on radiation emission

An object emits thermal radiation, carrying energy away from its surface. The rate of emission is the energy emitted as radiation per unit time.

Effect of surface temperature

As an object’s surface temperature increases, it emits thermal radiation at a greater rate. A hotter surface therefore transfers more energy away by radiation each second than the same surface at a lower temperature.

Effect of surface area

For surfaces at the same temperature with the same emitting properties, a larger surface area emits more radiation per unit time. Each unit of area emits at the same rate, so a larger area emits more in total. For example, doubling the area doubles the total emission rate under these conditions.

Emission and net radiative transfer

Emission transfers energy away from an object, but the object can also absorb radiation from its surroundings. Its net radiative energy transfer depends on both rates: emission carries energy away, while absorption brings energy in.

If emission exceeds absorption, the object has a net loss of energy by radiation. If absorption exceeds emission, it has a net gain. A higher emission rate alone does not determine the net transfer.

Comparing infrared emitters experimentally

Different surfaces can emit infrared radiation at different rates, even when they are at the same temperature. A Leslie cube allows their emitting abilities to be compared: its faces have different surface finishes, and the cube is filled with hot water to heat them.

Measuring the faces of a Leslie cube

Direct an infrared detector at one face and record its reading. Then measure each of the other faces in the same way. Compare the faces while they are at the same temperature.

A higher detector reading indicates a greater rate of infrared emission under the same measurement conditions. The face with the higher reading is therefore the better infrared emitter.

Making the comparison fair

Keep the detector the same distance from each face and point it at each face in the same orientation. The measured area must also be comparable, so that one reading is not higher simply because it includes more of the surface.

The faces must be at the same temperature because temperature also affects infrared emission. Keeping temperature and the measurement conditions comparable makes surface finish the relevant variable in the comparison.

Comparing infrared absorbers experimentally

Different surface finishes can absorb different amounts of infrared radiation. To compare them fairly, heat containers that differ only in their surface finish and measure how much their temperatures rise.

Comparing the surface finishes

1. Use identical containers with different surface finishes. Put equal amounts of the same material in each container and ensure they all start at the same temperature.
2. Place the containers the same distance from an infrared source, with each surface facing it at the same angle. Expose them to the source for the same length of time.
3. Measure the initial and final temperature of the material in each container. Calculate each temperature rise by subtracting the initial temperature from the final temperature.

Under these controlled conditions, a greater temperature rise indicates that the surface has absorbed more infrared radiation and is the better absorber.

Keep other conditions that affect energy loss as similar as possible, including the surroundings and airflow around the containers. This makes surface finish the main difference, so the temperature rises can be compared fairly.