One slit of a double-slit arrangement is covered by a thin glass plate with index of refraction 1.4 and the other by u thin glass plate with index of refraction 1.7. The point on the screen at which the central maximum fell before the glass plates were inserted is now occupied by what had been the m = 5 bright fringe. Assuming that λ = 480 nm and that the plates have the same thickness t, find t.