Monochromatic light of wavelength λ falls on two narrow slits separated by distance d, and the interference pattern is observed on a screen at distance D (with D≫d).
(i) Starting from the geometry of the experimental arrangement, derive an expression that relates the phase difference ϕ at a point located a distance y from the central maximum on the screen to the quantities y, d, D and λ.
(ii) Hence obtain a general expression for the separation Δy between two adjacent points on the screen where the phase difference differs by 2π.
(iii) Evaluate Δy numerically for λ=600nm, d=0.40mm and D=1.2m, and comment on what physical quantity Δy represents in an interference pattern.