Question

What is the emissivity of a black body?

Answer 1

Let us get some idea about the black body. A black body, also known as a blackbody, is a hypothetical physical body that absorbs all incoming electromagnetic radiation, regardless of frequency or incidence angle. Because it absorbs all hues of light, it is given the term "black body." Black-body radiation is also emitted by a black body. A white body, on the other hand, has a "rough surface that totally and equally reflects all incident rays in all directions."

Complete step-by-step solution:
Now let us understand Emissivity. In actuality, there are no flawless blackbodies in the real world. They are not perfect emitters of radiation because not all of the incident energy is absorbed. The relative ability of a material's surface to emit heat by radiation is measured by its emissivity ($\varepsilon$). The ratio of energy radiated from an object's surface to the energy radiated from a blackbody at the same temperature is known as emissivity.
$Emissivity(\varepsilon ) = \frac{{Radiant\,energy\,of\,an\,object\,at\,a\,given\,temperature}}{{Radiant\,energy\,of\,a\,blackbody\,at\,the\,same\,temperature}}$
The values of emissivity can range from $0$ to$1$. The emissivity of a blackbody is$1$, while that of a perfect reflector or whitebody is$0$. Because they release a portion of their maximum possible blackbody radiation at a given temperature, most natural objects are classified as "graybodies." The emissivity of water is near to one, and the emissivity of most vegetation is likewise close to one. The emissivities of many minerals and metals are much lower than$1$. The emissivity of a material can change depending on its temperature.

Note: The emissivity of a surface is determined not only by the material but also by its nature. A smooth and polished metal surface, for example, has a low emissivity, but a roughened and oxidised metal surface has a high emissivity.