Using monochromatic light of wavelength λ, an experimentalist sets up the Young's double slit experiment in three ways as shown. If she observes that y=β′, the wavelength of light used is
[JEE(Main) 2014, Online]
Diagram 1: Standard Young's double slit setup. Slits S1 and S2 are separated by distance d. The screen is at distance D. The central fringe is marked O. The fringe width is β.
Diagram 2: Young's double slit setup. Slit S1 is covered by a transparent mica sheet of thickness t=1.8μm and refractive index μ=1.6. Slits S1 and S2 are separated by distance d. The screen is at distance D. The central fringe is marked O. The fringe width is β. The distance y is shown from the central fringe O to the first bright fringe.
Diagram 3: Young's double slit setup. Slits S1 and S2 are separated by distance d. The screen is at distance 2D. The central fringe is marked O. The fringe width is β′.
(1) 580 nm
(2) 560 nm
(3) 520 nm
(4) 540 nm