JEE Main 2023 — Wave Optics Question with Solution

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). 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 $\left(\mathrm{\lambda }=4000 \mathrm{\AA }\right)$ from $S_1$ and $S_2$ respectively. The central bright fringe spot will shift by ________ number of fringes.