In a double slit arrangement, fringes are produced using light of wavelength $4800 \stackrel{\mathrm{o}}{\mathrm{A}}$. One slit is covered by a thin plate of glass of refractive index $1.4$ and the other with angular glass plate of the same thickness, but of refractive index $1.7$. By doing so, the central bright fringe shifts to original fifth bright fringe from the centre. Thickness of glass plate is