As shown in the figure, in Young's double slit experiment, a thin plate of thickness and refractive index is inserted infront of slit . The experiment is conducted in air () and uses a monochromatic light of wavelength . Due to the insertion of the plate, central maxima is shifted by a distance of . is the fringe-width before the insertion of the plate. The value of the is ________ . [2023]

(4)