A perfect black body is an object that absorbs all of the radiation…

The idea

A perfect black body is an idealised object that absorbs every bit of radiation that lands on it, at every wavelength. Nothing is reflected and nothing is transmitted, so the absorbed fraction is one. No real object is perfect, but a matt black surface comes close, and so does a small hole leading into a large cavity, because radiation entering the hole bounces around inside until it is absorbed. Stars are treated as black bodies, which shows that the name is about absorbing rather than about looking black: a star absorbs everything incident on it and is dazzling only because it is emitting so strongly at its own temperature. A perfect absorber is also the best possible emitter.

What earns the marks

• Defining a perfect black body as an object that absorbs all the radiation incident on it.
• Stating that none of the incident radiation is reflected.
• Stating that none of the incident radiation is transmitted.
• Naming a good approximation to a black body, such as a matt black surface or a star.

Common mistakes and how to put them right

• Restricting the definition to visible light rather than radiation of every wavelength.

• Insisting that a black body must look black even when it is very hot.

• Assuming that absorbing everything means the temperature must rise without limit.

• Naming a shiny black surface as a perfect black body.

Exam tips

• Build the definition from three ideas: absorbs all, reflects none, transmits none.
• If a question says stars act like black bodies, use it as the example rather than fighting it.
• Use 'radiation' rather than 'light' so the definition covers all wavelengths.