IGCSE Physics / Thermal energy
Thermal radiation: emission and absorption
Edexcel International GCSE Physics (4PH1), chapter 15: Thermal energy, specification point 4.6, 4.8. Free lesson.
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Thermal radiation transfers energy by infrared waves, so it can travel through a vacuum. All objects emit and absorb infrared, and the surface affects how quickly energy is transferred.
What thermal radiation is
Thermal radiation is energy transferred by infrared radiation, which is an electromagnetic wave. It does not need particles or any other medium, so it can travel through a vacuum. This is how energy from the Sun reaches Earth. Infrared radiation is the only way energy can cross empty space.
How temperature affects emission
All objects emit and absorb infrared radiation all the time, including cool objects. As an object's temperature rises, it emits more infrared radiation each second. A hotter object emits more infrared radiation each second.
How surfaces affect infrared
A dull black surface is the best emitter and absorber of infrared. A shiny surface is the poorest emitter and absorber, but it is a good reflector, so much of the infrared bounces off. A good absorber is also a good emitter, while a poor absorber is a poor emitter. At the same temperature, a dull black object cools faster than a shiny one because it emits infrared more effectively. Use the phrase “absorbs infrared radiation”. Heat itself is not something that is absorbed.
Why different surfaces are used
Designers choose surfaces according to whether they want energy to enter, leave or be reflected away. These choices affect how quickly an object gains or loses energy. Solar water panels are dull black, so they absorb as much of the Sun's infrared as possible and heat the water. Houses in hot countries are often painted white. White surfaces reflect sunlight, helping the houses stay cooler. A shiny silver survival blanket is a poor emitter and reflects infrared back towards the body, so the person loses less energy. A shiny teapot is a poor emitter, so the tea stays hot for longer.
Compare which can cool faster
To compare cooling, identify which surface emits infrared more effectively. The better emitter loses energy faster.