Young's double slit experiment is conducted with monochromatic light of wavelength \(5000\,\text{\AA}\), with slit separation of \(3\,\text{mm}\) and observer at \(20\,\text{cm}\) away from the slits. If a \(1\,\text{mm}\) transparent plate is placed in front of one of the slits, the fringes shift by \(6\,\text{mm}\). The refractive index of the transparent plate is