Define emissivity and absorptivity and give their values for a perfect black body.

(N/A) $1$. Absorptivity $(a)$: It is defined as the ratio of the amount of radiant energy absorbed by a body to the total amount of radiant energy incident on it. For a perfect black body,$a = 1$.
$2$. Emissivity $(e)$: It is defined as the ratio of the radiant energy emitted by a body at a given temperature to the radiant energy emitted by a perfect black body at the same temperature. For a perfect black body,$e = 1$.