Wave Optics Part 1

In Young's double slit experiment, two slits S1 and S2 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 0.1‌mm but refractive index 1.51 and 1.55 are introduced in the path of beam (λ=4000Å) from S1 and S2 respectively. The central bright fringe spot will shift by _______ number of fringes.