A body that absorbs the entire radiation energy incident on it is called as a perfect black body
For any perfectly black body , its absorption coefficient , a= 1
In practice , we use Ferry's body as perfectly black body as the lampblack which is used in it has a=0.97, i.e it absorbs almost 97% of the incident radiation.
Any perfect black body appears black in light and glows in dark because good absorber is a good emitter.
