For infinite parallel planes having emis-sivities $${{\varepsilon _1}}$$ & $${{\varepsilon _2}}$$, the interchange factor for radiation from surface 1 to surface 2 is given by

A. $$\frac{{{\varepsilon _1} + {\varepsilon _2}}}{{{\varepsilon _1}.{\varepsilon _2}}}$$

B. $$\frac{{{\varepsilon _1}.{\varepsilon _2}}}{{{\varepsilon _1} + {\varepsilon _2} - {\varepsilon _1}.{\varepsilon _2}}}$$

C. $${\varepsilon _1}.{\varepsilon _2}$$

D. $${\varepsilon _1} + {\varepsilon _2}$$

Answer: Option B