A flake of glass (refractive index 1.5) is placed over one of the openings of a double-slit apparatus. The interference pattern displaced itself through seven successive maxima toward the side where the flake is placed. If wavelength of the light is λ=600 nm , then the thickness of the flake is
(1) 2100 nm
(2) 4200 nm
(3) 8400 nm
(4) none of above