Black body

In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions.

"[1] A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic black-body radiation.

The radiation is emitted according to Planck's law, meaning that it has a spectrum that is determined by the temperature alone (see figure at right), not by the body's shape or composition.

[8]A more modern definition drops the reference to "infinitely small thicknesses":[9] An ideal body is now defined, called a blackbody.

A widely used model of a black surface is a small hole in a cavity with walls that are opaque to radiation.

[13] Typically, equilibrium is reached by continual absorption and emission of radiation by material in the cavity or its walls.

[14][15][16][17] Radiation entering the cavity will be "thermalized" by this mechanism: the energy will be redistributed until the ensemble of photons achieves a Planck distribution.

A body's behavior with regard to thermal radiation is characterized by its transmission τ, absorption α, and reflection ρ.

[22] A few idealized types of behavior are given particular names: An opaque body is one that transmits none of the radiation that reaches it, although some may be reflected.

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

[25] Kirchhoff in 1860 introduced the theoretical concept of a perfect black body with a completely absorbing surface layer of infinitely small thickness, but Planck noted some severe restrictions upon this idea.

As a perfect emitter of radiation, a hot material with black body behavior would create an efficient infrared heater, particularly in space or in a vacuum where convective heating is unavailable.

Nano-porous materials can achieve refractive indices nearly that of vacuum, in one case obtaining average reflectance of 0.045%.

[5][35] In 2009, a team of Japanese scientists created a material called nanoblack which is close to an ideal black body, based on vertically aligned single-walled carbon nanotubes.

[40] Combining the U (ultraviolet) and the B indices leads to the U-B index, which becomes more negative the hotter the star and the more the UV radiation.

Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return.

[51] The Big Bang theory is based upon the cosmological principle, which states that on large scales the Universe is homogeneous and isotropic.

The cosmic microwave background radiation observed today is "the most perfect black body ever measured in nature".

[52] It has a nearly ideal Planck spectrum at a temperature of about 2.7 K. It departs from the perfect isotropy of true black-body radiation by an observed anisotropy that varies with angle on the sky only to about one part in 100,000.

The integration of Planck's law over all frequencies provides the total energy per unit of time per unit of surface area radiated by a black body maintained at a temperature T, and is known as the Stefan–Boltzmann law: where σ is the Stefan–Boltzmann constant, σ ≈ 5.67×10−8 W⋅m−2⋅K−4‍[53] To remain in thermal equilibrium at constant temperature T, the black body must absorb or internally generate this amount of power P over the given area A.