Black body

A black body or blackbody is an idealized <a href="/facts/Physical_object/WhqjK6xo">physical body</a> that <a href="/facts/Absorption_(electromagnetic_radiation)/xBYKt7yS">absorbs</a> all incident <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a>, regardless of frequency or <a href="/facts/Angle_of_incidence_(optics)/DTaCjhTU">angle of incidence</a>. The radiation emitted by a black body in thermal equilibrium with its environment is called <a href="/facts/Black-body_radiation/K4YKPKXL">black-body radiation</a>. The name "black body" is given because it absorbs all colors of light. In contrast, a white body is one with a "rough surface that reflects all incident rays completely and uniformly in all directions."
A black body in <a href="/facts/Thermal_equilibrium/wHqArIwx">thermal equilibrium</a> (that is, at a constant temperature) emits electromagnetic black-body radiation. The radiation is emitted according to <a href="/facts/Planck%27s_law/7Umkdb9m">Planck's law</a>, meaning that it has a <a href="/facts/Frequency_spectrum/q3Rfs7b0">spectrum</a> that is determined by the <a href="/facts/Temperature/QyGe4gmj">temperature</a> alone (see figure at right), not by the body's shape or composition.
An ideal black body in thermal equilibrium has two main properties:

<ol><li>It is an ideal emitter: at every frequency, it emits as much or more thermal radiative energy as any other body at the same temperature.</li>
<li>It is a diffuse emitter: measured per unit area perpendicular to the direction, the energy is radiated <a href="/facts/Isotropic_radiator/HFGgrM74">isotropically</a>, independent of direction.</li></ol>
Real materials emit energy at a fraction—called the <a href="/facts/Emissivity/LUxfMRRI">emissivity</a>—of black-body energy levels. By definition, a black body in thermal equilibrium has an emissivity ε = 1. A source with a lower emissivity, independent of frequency, is often referred to as a gray body.
Constructing black bodies with an emissivity as close to 1 as possible remains a topic of current interest.
In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, the radiation from <a href="/facts/Star/PFbd6ICx">stars</a> and <a href="/facts/Planet/rbPjE6Bx">planets</a> is sometimes characterized in terms of an <a href="/facts/Effective_temperature/RY30DaSY">effective temperature</a>, the temperature of a black body that would emit the same total flux of electromagnetic energy.

Black body open-in-new

Black body