Q.Define 'emissive power' and 'coefficient of emission of a body'.

Emissive power measures how much a body radiates; the coefficient of emission compares that to the ideal black-body radiator.

Emissive power ($e$): The emissive power of a body, at a given temperature, is defined as the total amount of thermal radiant energy emitted per unit time, per unit surface area of the body, considering radiation of all wavelengths. Its SI unit is $\text{J s}^{-1}\text{m}^{-2}$ or $\text{W/m}^2$.

Coefficient of emission (emissivity, $e$ or $\varepsilon$) of a body: It is defined as the ratio of the emissive power of the given body ($E$) to the emissive power of a perfectly black body ($E_b$) at the same temperature:

$\varepsilon=\frac{E}{E_b}$

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