Thermal radiation

All bodies — objects — no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time.

The phrase "no matter what temperature" is worth pausing on. A cold object still emits infrared; it simply emits less than a hot one. Nothing has to be glowing, or even warm, to be radiating.

A perfect black body is an object that absorbs all of the radiation incident on it. A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.

You should be able to explain:

• that all bodies (objects) emit radiation • that the intensity and wavelength distribution of any emission depends on the temperature of the body.

Two things change together as a body gets hotter. The intensity increases — it radiates more energy in a given time. And the wavelength distribution shifts, so that more of the radiation is emitted at shorter wavelengths.

That shift is why a piece of metal heated in a flame first radiates only infrared, which you feel but cannot see, then glows dull red, then orange, then white as it gets hotter still. The peak of its emission has moved through the visible range.

A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation.

So temperature is not about how much radiation arrives, but about the balance between what arrives and what leaves:

• absorbing faster than emitting — the temperature rises • emitting faster than absorbing — the temperature falls • the two rates equal — the temperature stays constant.

And because a hotter body emits more, a body warming up moves towards balance on its own: as its temperature rises, its rate of emission rises until it matches the rate of absorption, and the temperature then settles.

The temperature of the Earth depends on many factors including the rates of absorption and emission of radiation, and reflection of radiation into space.

You should be able to explain how the temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted, using everyday examples to illustrate this balance, and the example of the factors which determine the temperature of the Earth. You should also be able to use information, or draw and interpret diagrams, to show how radiation affects the temperature of the Earth's surface and atmosphere.

The Earth's balance, in order:

1. Radiation arrives from the Sun, mostly at short wavelengths. 2. Some is reflected straight back into space by clouds, ice and light-coloured surfaces, and never warms anything. 3. The rest is absorbed by the surface, which warms up. 4. The warmed surface emits radiation of its own, at longer wavelengths — infrared — because it is far cooler than the Sun. 5. Some of that infrared escapes to space; some is absorbed by gases in the atmosphere and re-emitted, partly back towards the surface. 6. The Earth's temperature settles where the rate of emission to space equals the rate of absorption from the Sun.

Anything that changes one side of that balance changes the temperature. More reflective surfaces, such as more ice, mean less absorbed and a cooler Earth. More gases absorbing the outgoing infrared mean less escaping to space, so the Earth must warm until the rates balance again.