**Emissive Power,
Absorptive Power and Emissivity**

If temperature of a body is more than its surrounding then body emits thermal radiation.

**1) Monochromatic Emittance or Spectral
emissive power\(\left( {{e}_{\lambda }} \right)\): **For a given
surface it is defined as the radiant energy emitted per sec per unit area of
the surface with in a unit wavelength around \(\lambda \) i.e. lying between \(\left(
\lambda -\frac{1}{2} \right)\) to\(\left(
\lambda +\frac{1}{2} \right)\).

Spectral emissive power\(\left( {{e}_{\lambda }} \right)=\frac{Energy}{Area\times Time\times Wavelength}\)

**2) Total Emittance (or) Total Emissive
Power\(\left( e \right)\): **It is defined as
the total amount of thermal energy emitted per unit time, per unit area of the
body for all possible wavelengths.

**3) Monochromatic absorptance or
spectral absorptive power\(\left( {{a}_{\lambda }} \right)\): **It is defined as
the ratio of the amount of the energy absorbed in a certain time to the total
heat energy incident upon it in the same time, both in the unit wavelength
interval. It is dimensionless and unit less quantity.

**4) Total absorptance (or) Total
absorpting power\(\left( a \right)\): **It is defined as
the total amount of thermal energy absorbed per unit time, per unit area of the
body for all possible wavelengths.

**5) Emissivity**\(\left(\varepsilonÂ \right)\): Emissivity of a body at a given temperature is defined as the ratio of the total emissive power of the body\(\left( e \right)\) to the total emissive power of a perfect black body\(\left( E \right)\) at that temperature, i.e. \(\varepsilon =\frac{e}{E}\)

(i) For perfectly black body, \(\varepsilon =1\)

(ii) For highly polished body, \(\varepsilon =0\)

(iii) For practical bodies emissivity \(\left( \varepsilonÂ \right)\) lies between zero and one \(\left( 0<\varepsilon <1 \right)\)

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