Black Body Radiation and Emission of Radiation — AQA GCSE Physics (Separate)
All objects emit and absorb radiation. A perfect black body is the ideal emitter and absorber.
Emitting and absorbing radiation
All objects emit and absorb infrared radiation. The hotter an object, the more radiation it emits in a given time.
- Matt black surfaces are the best emitters and absorbers.
- Shiny, light-coloured surfaces are good reflectors and poor absorbers/emitters.
Perfect black body
A perfect black body is an object that absorbs all the radiation that hits it (it reflects and transmits none). Because it is a perfect absorber, it is also the best possible emitter of radiation.
Temperature and radiation
As an object's temperature increases:
- it emits more radiation overall, and
- the peak wavelength of the emitted radiation gets shorter (e.g. a metal glowing red, then orange, then white-hot).
Earth's temperature
The temperature of an object (like the Earth) depends on the balance between the radiation it absorbs and the radiation it emits:
- If it absorbs more than it emits, its temperature rises.
- If it emits more than it absorbs, its temperature falls.
- A constant temperature means absorption and emission are equal.
Changes to this balance (e.g. by greenhouse gases) affect Earth's average temperature.
Exam tips
- Hotter objects emit more radiation and at a shorter peak wavelength.
- A perfect black body absorbs all radiation and is the best emitter.
- Matt black = good absorber/emitter; shiny = good reflector/poor absorber.
- Earth's temperature depends on the balance of radiation absorbed and emitted.