A wedge-shaped film of air is produced by placing a fine wire of diameter D between the ends of two flat glass plates (not thin) of Length L = 20 cm, as shown in the figure. When the air film is illuminated with light of wavelength $\lambda =550nm$ for near  normal incident there are 12 dark fringes per centimeter in reflected light. $D=\left(1.1\right)n\times {10}^{-5}m$  then n is