Editing Black body (section)

=== Transmission, absorption, and reflection ===
A body's behavior with regard to thermal radiation is characterized by its transmission ''τ'', absorption ''α'', and reflection ''ρ''.
 
The boundary of a body forms an interface with its surroundings, and this interface may be rough or smooth. A nonreflecting interface separating regions with different refractive indices must be rough, because the laws of reflection and refraction governed by the [[Fresnel equations]] for a smooth interface require a reflected ray when the refractive indices of the material and its surroundings differ.<ref name=Tipler/> A few idealized types of behavior are given particular names:

An ''[[Opacity (optics)|opaque body]]'' is one that transmits none of the radiation that reaches it, although some may be reflected.<ref name=Kaviany/><ref name=Venkanna/> That is, ''τ'' = 0 and ''α'' + ''ρ'' = 1.

A ''[[Transparency (optics)|transparent body]]'' is one that transmits all the radiation that reaches it. That is, ''τ'' = 1 and ''α'' = ''ρ'' = 0.

A '''''grey body''''' is one where ''α'', ''ρ'' and ''τ'' are constant for all wavelengths; this term also is used to mean a body for which ''α'' is temperature- and wavelength-independent.

A ''white body'' is one for which all incident radiation is reflected uniformly in all directions: ''τ'' = 0, ''α'' = 0, and ''ρ'' = 1.

For a black body, ''τ'' = 0, ''α'' = 1, and ''ρ'' = 0. Planck offers a theoretical model for perfectly black bodies, which he noted do not exist in nature: besides their opaque interior, they have interfaces that are perfectly transmitting and non-reflective.<ref>{{harvnb|Planck|1914|page=10}}</ref>