Surfaces and radiated heat
Objects give out and take in heat by radiation. How well a surface does this depends on its colour and texture. The two things a surface does are absorbing heat that lands on it and emitting (giving out) heat.
Dull black surfaces
A dull, dark, rough surface is the best at both absorbing and emitting heat radiation. It heats up quickly in the Sun and also cools down quickly because it gives out heat well.
Key idea
Dull black surfaces are good absorbers and good emitters of heat. Shiny white (silvery) surfaces are poor absorbers and poor emitters; they reflect heat instead.
Shiny white surfaces
A shiny, light-coloured, smooth surface is a poor absorber and poor emitter of heat radiation. Instead of soaking heat up, it reflects most of it away, so it stays cooler in the Sun.
Everyday uses
- People wear light-coloured clothes in hot weather because they absorb less heat.
- Teapots and kettles are often shiny to reduce heat loss and keep drinks hot longer.
- The backs of refrigerators and solar water heaters are painted dull black to emit or absorb heat well.
Example
On a sunny day, a black car becomes much hotter inside than a white car, because the black surface absorbs more heat radiation.
Remember
- Dull black: good absorber and good emitter.
- Shiny white: poor absorber and poor emitter, good reflector.
- Choose the surface to suit the job: keeping cool or losing heat.