<P> Suppose the cavity is held at a fixed temperature T and the radiation trapped inside the enclosure is at thermal equilibrium with the enclosure . The hole in the enclosure will allow some radiation to escape . If the hole is small, radiation passing in and out of the hole has negligible effect upon the equilibrium of the radiation inside the cavity . This escaping radiation will approximate black - body radiation that exhibits a distribution in energy characteristic of the temperature T and does not depend upon the properties of the cavity or the hole, at least for wavelengths smaller than the size of the hole . See the figure in the Introduction for the spectrum as a function of the frequency of the radiation, which is related to the energy of the radiation by the equation E = hf, with E = energy, h = Planck's constant, f = frequency . </P> <P> At any given time the radiation in the cavity may not be in thermal equilibrium, but the second law of thermodynamics states that if left undisturbed it will eventually reach equilibrium, although the time it takes to do so may be very long . Typically, equilibrium is reached by continual absorption and emission of radiation by material in the cavity or its walls . Radiation entering the cavity will be "thermalized"; by this mechanism: the energy will be redistributed until the ensemble of photons achieves a Planck distribution . The time taken for thermalization is much faster with condensed matter present than with rarefied matter such as a dilute gas . At temperatures below billions of Kelvin, direct photon--photon interactions are usually negligible compared to interactions with matter . Photons are an example of an interacting boson gas, and as described by the H - theorem, under very general conditions any interacting boson gas will approach thermal equilibrium . </P> <P> A body's behavior with regard to thermal radiation is characterized by its transmission τ, absorption α, and reflection ρ . </P> <P> 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 . A few idealized types of behavior are given particular names: </P>

The fraction of incident light a body reflects is known are