A thin sheet of glass (refractive indeed 1.5) of thickness 6 microns, introduced in the path of one of the interfering beams in a double-slit experiment, shifts the central fringe to a position earlier occupied by the fifth bright fringe. The wavelength of light used is
3000`stackrel(o)(A)`
6000`stackrel(o)(A)`
4500`stackrel(o)(A)`
7000`stackrel(o)(A)`