Question

Emissivity of a perfect black body is

• A. always $0$.
• B. always $1$.
• C. between $0$ and $1$.
• D. always $>1$.

Answer 1

The correct option is B always $1$.

Emissivity of a perfect black body is always 1.

The best absorber is defined as the object which can absorb all the electromagnetic radiations falling upon it. The black body is not only a perfect absorber but it is also the best in emitting radiation. Also, a black bosy in thermal equlibrium has emissivity, $ϵ=1$