In a double-slit arrangement, fringes are produced using light of wavelength 4800 A. One slit is covered by a thin plate of glass of refractive index 1. 4 and other by another plate of glass of the same thickness but of refractive index 1.7. On doing so, the central bright fringe shifts to the position originally occupied by the fifth bright fringe from the central maximum The thickness of  glass plate is ………….