The radiation which leaves surface 1, the amount that reaches surface 2 is

J₁A₁F₁₂

Similarly, the radiation which leaves system 2, the amount that reaches surface 1 is

J₂A₂F₂₁

The net energy transfer between the surfaces,

Reciprocity theorem states that

(7.27)

It also resembles an electrical circuit shown in fig.7.7b. The difference between eq.7.26 and 7.27 is that in eq.7.27 the denominator term is space resistance instead of surface resistance.

Now, to know, the net energy exchange between the two surfaces we need to add both the surface resistances along with the overall potential as shown in the fig.7.8. Here the surfaces see each other and nothing else.

Fig. 7.8: Radiative nature for two surfaces which can see each other nothing else

(7.28)