Perfect Blackbody

A body that absorbs the entire radiant energy incident on it -- i.e. $a = 1$ in the notation of Section 3.10.1 -- is called an ideal or perfect blackbody. Because such a surface reflects none of the light falling on it, it appears black to the eye (unless it is itself so hot that it glows / is self-luminous). Real-world surfaces such as lamp black or platinum black, which absorb around 97% of incident radiant heat, are good practical approximations to a perfect blackbody.

Why good absorbers are good emitters. Consider two objects, opaque to thermal radiation, of identical temperature and identical surface area: one well-polished (a good reflector, poor absorber), the other painted matte black (a good absorber, poor reflector). For each object to maintain its temperature in a steady environment, its rate of emission of thermal radiation must exactly balance its rate of absorption. Since the black-painted object absorbs far more radiation than the polished one, it must also emit far more, purely to keep its temperature constant. This reasoning generalises: good absorbers are always good emitters, and poor absorbers are poor emitters. Because every object must either absorb or reflect whatever radiation is incident on it, a poor absorber is necessarily a good reflector, and therefore a poor emitter -- which is precisely why the walls of a vacuum flask (thermos) are silvered: a highly reflective (and therefore poor-emitting) surface minimises radiative heat loss from the hot liquid inside. …