Infrared Radiation and Emission/Absorption — AQA GCSE Physics
All objects emit and absorb infrared radiation. The rate depends on temperature and surface.
Infrared radiation
Infrared is part of the electromagnetic spectrum, between microwaves and visible light. All objects emit and absorb infrared radiation continuously. The hotter an object, the more infrared it emits per second.
Surfaces and radiation
- Matt black surfaces are the best emitters and the best absorbers of infrared.
- Shiny, light-coloured (e.g. silver) surfaces are good reflectors and poor absorbers/emitters.
This is why, for example, the cooling fins on the back of a fridge are often painted black (to emit heat well), and survival blankets are shiny (to reflect heat back).
Required practical
You can investigate infrared radiation using a Leslie cube — a metal cube with different surfaces (matt black, shiny, white) filled with hot water. An infrared detector shows that the matt black surface emits the most radiation.
Temperature balance
An object that emits more radiation than it absorbs cools down; one that absorbs more than it emits warms up; equal rates keep the temperature constant.
Exam tips
- All objects emit and absorb infrared; hotter objects emit more.
- Matt black = best emitter/absorber; shiny = good reflector/poor absorber.
- Know the Leslie cube practical.
- Temperature changes depend on the balance of emission and absorption.