is: an idealized object that absorbs all the electromagnetic radiation that falls upon it. [P8.2, P10.1]
reflects: absolutely no radiation. [P8.2, P10.1]
is also: an ideal emitter of radiation. [P8.2, P10.1]
emits: black-body radiation - which has a spectrum that depends only on the temperature of the black body. [P8.2, P10.1]
has: spectral brightness which is given by Planck's function. [P7.3]
is approximated roughly: by a matt black surface. [P7.3]
is approximated well: by a cavity maintained at a well-defined temperature and connected to its environment by a small aperture. The spectrum of radiation inside such a cavity is described quite accurately by Planck's function, and the radiation emitted from the small hole closely approximates black-body radiation irrespective of the material of the container or the state of its inner surface. [P7.3, P8.2, P10.1]
Copyright 1997, The Open University