A monochromatic beam of light falls on YDSE apparatus at some angle (say θ) as shown in figure. A thin sheet of glass is inserted in front of the lower slit S₂. The central bright fringe (path difference = 0) will be obtained

Anywhere depending on angle θ, thickness of plate t and refractive index of glass µ

If dsinθ = (µ −1)t, central fringe is obtained at O

If dsinθ > (µ −1)t, central fringe is obtained above O and

If dsinθ < (µ −1)t, central fringe is obtained below O.