Infrared and black body radiation

All objects give out (emit) and take in (absorb) infrared radiation, whatever their temperature. Even ice emits some.

The hotter an object is, the more infrared it emits in a given time.

How much an object emits and absorbs also depends on its surface:

• Matt black surfaces are the best emitters and the best absorbers.
• Shiny silver or white surfaces are the worst emitters and absorbers. They are good reflectors.

That's why cooling fins on the back of a fridge are black, and why emergency blankets are shiny.

A perfect black body is an object that absorbs all the radiation that hits it. It doesn't reflect or transmit any.

A good absorber is also a good emitter, so a perfect black body is the best possible emitter too.

An object stays at a constant temperature when it absorbs radiation at the same rate as it emits it.

• If it absorbs faster than it emits, its temperature rises.
• If it emits faster than it absorbs, its temperature falls.

As an object gets hotter, it emits more radiation at every wavelength, but the shorter wavelengths increase fastest. That's why metal glows red, then yellow, then white as it heats up.

The Earth's temperature depends on the balance between the radiation it absorbs and the radiation it emits:

• The Sun's radiation, mostly visible light, reaches the Earth. Some is reflected; the rest is absorbed by the surface, warming it.
• The warm surface emits infrared. Some escapes into space; some is absorbed by gases in the atmosphere, such as carbon dioxide, and emitted back towards the surface.

In the day, the Earth absorbs more than it emits, so it warms up. At night, it emits more than it absorbs, so it cools. Over a whole year, the two roughly balance.

A Leslie cube is a hollow metal cube with four different faces: matt black, shiny black, matt white and shiny silver.

1. Fill the cube with hot water from a kettle and put on the lid.
2. Point an infrared detector at one face, from a measured distance such as 10 cm.
3. Record the reading. Repeat for each face, at the same distance.

Result: the matt black face emits the most infrared; the shiny silver face emits the least.

• Independent variable: the type of surface.
• Dependent variable: the amount of infrared emitted (the detector reading).
• Control variables: the distance from the face and the water temperature.

Hazard: hot water and the hot cube can scald, so handle with care.

Improving accuracy: take the readings quickly, so the water doesn't cool between faces.