One slit of a double slits experiment is covered by a thin glass plate of refractive index`1.4`and the other by a thin glass plate of refractive index`1.7` .The point on the screen ,where central bright fringe was formed before the introduction of the glass sheets,is now occupied by the `15th`bright fringe.Assuming that both the glass plates have same thickness and wavelength of light used in `4800Å` ,find the their thickness.
