The interchange factor for radiation heat transfer from surface 'x' to surface 'y' in case of an infinite parallel planes with emis-sivities $${\varepsilon _{\text{x}}}$$ & $${\varepsilon _{\text{y}}}$$ is given by

A. $${\varepsilon _{\text{x}}} + {\varepsilon _{\text{y}}}$$

B. $${\varepsilon _{\text{x}}} \cdot {\varepsilon _{\text{y}}}$$

C. $$\frac{1}{{{\varepsilon _{\text{x}}}}} + \frac{1}{{{\varepsilon _{\text{y}}}}}$$

D. $$\frac{{{\varepsilon _{\text{x}}} + {\varepsilon _{\text{y}}}}}{{{\varepsilon _{\text{x}}} + {\varepsilon _{\text{y}}} - {\varepsilon _{\text{x}}} \cdot {\varepsilon _{\text{y}}}}}$$

Answer: Option D