Core practical: thermal radiation and the Earth's temperature

Every object gives out and takes in infrared radiation. The balance between the two decides whether it warms up, cools down or stays the same.

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Emitting and absorbing

All objects emit and absorb infrared radiation.
The hotter an object, the more radiation it emits per second, and the shorter the wavelengths (very hot objects also give out visible light).
If an object absorbs radiation faster than it emits it, its temperature rises. If it emits faster than it absorbs, it cools.
At a constant temperature, it absorbs and emits radiation at the same rate.

The Earth's temperature

The Earth absorbs radiation from the Sun and emits infrared radiation into space. Its temperature depends on the balance.
Some radiation is reflected by clouds, ice and snow.
Greenhouse gases absorb some of the infrared emitted by the Earth, so more energy stays in the atmosphere. More greenhouse gas means the balance happens at a higher temperature.

Core practical: different surfaces

1. Fill containers with different surfaces (matt black, shiny black, matt white, shiny silver) with the same volume of hot water at the same starting temperature. Put on lids.
2. Record the temperature of each every minute with a thermometer (or measure the radiation with an infrared detector at the same distance).
Results: the matt black surface cools fastest (best emitter); the shiny silver surface cools slowest (worst emitter).
Good emitters are also good absorbers. Shiny surfaces are good reflectors.
Worked example

Explain why a kettle with a shiny silver outside keeps water hot for longer than a matt black kettle.

  1. Shiny silver surfaces are poor emitters of infrared radiation.
  2. So energy is transferred away from the water more slowly.
  3. The water stays hotter for longer.

Answer: Shiny surfaces emit infrared radiation more slowly than matt black ones, so less energy is lost per second.

Key idea

Hotter objects emit more radiation and shorter wavelengths. Absorbing faster than emitting makes an object warm up; at constant temperature they are equal. Matt black surfaces are the best emitters and absorbers; shiny surfaces are the worst and reflect radiation.

Key words
Temperature
K = °C + 273. A change of 1 °C is the same as a change of 1 K.
Infrared
Electromagnetic radiation just beyond red light. Emitted by all objects; hotter objects emit more.

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Extra practice

2 more questions

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  1. Which surface is the worst absorber of infrared radiation?

    1. Dull grey
    2. Dark green
    3. Shiny silver
    4. Matt black
    Show the answer

    Answer: Shiny silver

    Shiny surfaces reflect most radiation, so they absorb little.

  2. An object is warming up. What can you say about the radiation it absorbs and emits?

    1. It absorbs and emits at the same rate
    2. It emits faster than it absorbs
    3. It emits no radiation
    4. It absorbs radiation faster than it emits it
    Show the answer

    Answer: It absorbs radiation faster than it emits it

    A temperature rise means more energy in than out.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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