In Young's double slit experiment, two slits $\mathrm{S}_1$ and $\mathrm{S}_2$ are 'd' distance apart and the separation from slits to screen is D (as shown in figure). Now if two transparent slabs of equal thickness 0.1 mm but refractive index 1.51 and 1.55 are introduced in the path of beam $(\lambda=4000 \A)$ from $\mathrm{S}_1$ and $\mathrm{S}_2$ respectively. The central bright fringe spot will shift by $\_\_\_\_$ number of fringes.