In a young's double slit arrangement fringes are produced using light of wavelength 4000 $$\mathop A\limits^o $$. One slit is covered by a thin plate of glass of refractive index 1.4 and the other with another glass plate of same thickness but of refractive index 1.7. By doing so the central bright shifts to original sixth fringe from centre. Thickness of glass plate is ................. .