The best absorber is defined as the object which can absorb all the electromagnetic radiations falling upon it. It is closer to a real object than an ideal black body, since it may have absorptivity less than 1 and reflectivity and transmissivity greater than 0. A black body is a material that is a perfect emitter of heat energy and has an emissivity … In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium. The black body is not only a perfect absorber but is also the best in emitting radiation. A body at temperature T radiates electromagnetic energy.
and absorptivity is the amount of heat absobed by body devided by the incident heat energy .but my question is ,are these emissivity and absorptivity constant or depends on temperature? The surface of a blackbody emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K). emissivity tells about the ability to emit heat energy as thermal radiation compare to a black body. A law closely related to the blackbody is Kirchhoff’s law that defines reflection, transmission, absorption and radiation. THERMAL EMISSIVITY AND RADIATIVE HEAT TRANSFER (CONTINUED) A gray body is the term for a non-existent, ideal body that has the same value of emissivity at all wavelengths. Kirchhoff's law Such an object is often called a black body. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation.

The maximum possible emissivity of 1 would imply that incident radiation is completely absorbed, and for a given temperature the object then emits thermal radiation with the maximum strength.
Thermal radiation is electromagnetic radiation that may include both visible radiation and infrared radiation, which is not visible to human eyes. I have read the radiation chapter, where I have been introduced with the terms emissivity and absorptivity. A high emissivity of a material comes together with a high absorptance. Black Body Type and Emissivity Although a blackbody is actually only a theoretical ideal, an object can be formed which approximates it.

The thermal radiation from very hot objects is easily visible to the eye. A perfect black body in thermodynamic equilibrium absorbs all light that strikes it, and radiates energy according to a unique law of radiative emissive power for temperature T, universal for all perfect black bodies. Emissivity is simply a factor by which we multiply the black body heat transfer to take into account that the black body is the ideal case.