A monochromatic light source S of wavelength 440 nm is placed slightly above a plane mirror M as shown . Image of S in M can be used as a virtual source to produce interference fringes on the screen. The distance of source S from O is 20.0 cm, and the distance of screen from O is `100.0` cm (figure is not to scale). If the angle `theta = 0.50 xx10^(-3)` radians, the width of the interference fringes observed on the screen is –