All bodies (objects), no matter what temperature, emit and absorb infrared…

The idea

Every object emits infrared radiation all the time, whatever its temperature, because its particles have internal energy; even ice emits infrared. At the same moment it is absorbing infrared coming from its surroundings, so emission and absorption always happen together and it is the balance between them that decides whether the temperature changes. How well a surface does either depends on its colour and its texture. In the AQA cube experiment four cubes of the same size were placed the same distance from an infrared heater: matt black rose 6.5 °C, shiny black 4.0 °C, matt white 3.0 °C and shiny white only 2.0 °C, showing that black absorbs more than white and that matt absorbs more than shiny of the same colour.

What earns the marks

• Stating that all objects emit infrared radiation whatever their temperature.
• Stating that objects absorb infrared radiation from their surroundings at the same time.
• Concluding that black surfaces absorb more infrared than white surfaces.
• Concluding that matt surfaces absorb more infrared than shiny surfaces of the same colour.

Common mistakes and how to put them right

• Writing a conclusion such as 'black absorbs more' without saying more than what.

• Stating that only hot objects emit infrared radiation.

• Saying cold objects absorb radiation but never emit any.

• Treating the measured temperature increase as the radiation the cube emitted rather than absorbed.

Exam tips

• Give two separate conclusions when asked for two: one about colour, one about finish.
• Quote the temperature figures as support for each conclusion.
• Keep the control variables in mind: same size cube, same distance, same time.