 ## 9. Kirchhoff's law of Thermal Radiation

• Good absorbers of radiation are also good radiators this statement is quantitatively explained by Kirchhoff's law.
(i) Emissive Power -
Emissive power denotes the energy radiated per unit area per unit solid angle normal to the area.

E = Δu/ [(ΔA) (Δω) (Δt)]
where, Δu is the energy radiated by area ΔA of surface in solid angle Δω in time Δt.

(ii) Absorptive Power -
Absorptive power of a body is defined as the fraction of the incident radiation that is absorbed by the body
a(absorptive power) = energy absorbed / energy incident

(iii) Kirchhoff's Law
"It status that at any given temperature the ratio of emissive power to the absorptive power is constant for all bodies and this constant is equal to the emissive power of perfect B.B. at the same temperature.
E/abody=EB.B.
• From kirchhoff's law we can say that a body having high emissive power should have high absorptive power and those having low emissive power should have law absorptive power so as to keep the ratio E/a same.