Interference fringes were produced with white light by a double slit arrangement. When a mica sheet of uniform thickness of refractive index 1.6 (relative to air) is placed in the path of light from one of the slits, the central fringe moves through a distance equal to the width of 30 interference bands if light of wavelength $4800 \stackrel{\mathrm{o}}{\mathrm{A}}$ is used. The thickness (in 10^–6m) of mica is 4n. find n.