1. The problem statement, all variables and given/known data Young's double slit experiment 'd' - Separation between the two slits 'D' - Separation between the double slit and the screen 'S' - Source (Primary) (It is coherent) S1, S2 - Secondary sources Δx - Path difference between the two rays coming out from S1 and S2 Δx° - Path difference between the two rays coming out from S1 and S2 when a glass slab with refractive index μ and thickness 't' is placed in front of one of the slits. λ - wavelength of the light waves d<<D 2. Relevant equations What my teacher did was, He said that the path difference when the glass slab is kept in front of the slit would be, Δx° = μt-t And then he wrote, Δx° = d sinθ - (μt-t) And then he did the usual calculations and, finally, got y = [nλ + (μ-1)t]D/d I understood all the calculations he did. But I'm confused because he didn't consider the 'refraction' of light rays that pass through the slab anywhere while deriving the expression. Won't the rays bend? Can the lateral displacement be neglected?