As shown in the figure, in Young's double slit experiment, a thin plate of thickness `t = 10 mu m` and refractive index `mu= 1.2 `is inserted infront of slit `S_(1)`. The experiment is conducted in air `(mu= 1)` and uses a monochromatic light of wavelength `lambda= 500 nm`. Due to the insertion of the plate, central maxima is shifted by a distance of `xbeta_0. beta_0` is the fringe–width before the insertion of the plate. The value of the `x` is ______________