For infinite parallel planes having emissivities ε1 & ε2, the interchange factor for radiation from surface 1 to surface 2 is given by__________________?

(A) (ε1 + ε2)/ε1.ε2 (X)
(B) ε1.ε2/(ε1 + ε2 – ε1.ε2) ()
(C) ε1.ε2 (X)
(D) ε1 + ε2 (X)

Answers:  For infinite parallel planes having emissivities ε1 & ε2, the interchange factor for radiation from surface 1 to surface 2 is given by ε1.ε2/(ε1 + ε2 – ε1.ε2).