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 D (as shown in figure).…

In Young's double slit experiment, two slits $\displaystyle \mathrm{S}_1$ and $\displaystyle \mathrm{S}_2$ 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 $\displaystyle 0.1$ mm but refractive index $\displaystyle 1.51$ and $\displaystyle 1.55$ are introduced in the path of beam $\displaystyle (\lambda=4000 \AA)$ from $\displaystyle \mathrm{S}_1$ and $\displaystyle \mathrm{S}_2$ respectively. The central bright fringe spot will shift by $\displaystyle \_\_\_\_$ number of fringes.