Kirchoff's Law for black body

This law states that at a given temperature, the ratio of emission power to the absorption power for a given wavelength is same for all bodies and is equal to the emissive power of a black body at the same temperature.

Energy emitted per square meter is eƛdƛ.
Energy absorbed per unit area is QƛdQ.

eƛ/aƛ = ∂Qƛ /∂ƛ = constant

where eƛ is emission power ; aƛ is absorptive power
∂Qƛ is energy falling on unit area per second within same wavelength range & temperature.
(∂Qƛ /∂ƛ) depends only on temperature and for a given temperature it is constant.

For a perfectly black body, aƛ = 1

Note: Kirchoffs law is true only for temperature radiation.

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