Wave Optics

In Young's double slit experiment, two slits \(S_{1}\) and \(S_{2}\) are '\(d\)' distance apart and the separation from slits to screen is \(\mathrm{D}\) (as shown in figure). Now if two transparent slabs of equal thickness \(0.1 \mathrm{~mm}\) but refractive index \(1.51\) and \(1.55\) are introduced in the path of beam \((\lambda=4000\) \(\mathop A\limits^o\)) from \(\mathrm{S}_{1}\) and \(\mathrm{S}_{2}\) respectively. The central bright fringe spot will shift by ___________ number of fringes.