The view factor F21 for radiation heat transfer is

Question

A solid sphere of radius r₁=20mm is placed concentrically inside a hollow sphere of radius r₂ = 30 mm as shown in the figure.

The view factor F₂₁ for radiation heat transfer is                                  

      (a) 2/3                             (b) 4/9                          (c) 8.27                           (d) 9/4