Young's double slit experiment gives interference fringes of width $0.3 \mathrm{~mm}$. A thin glass plate made of material of refractive index $1.5$ is kept in the path of light from one of the slits, then the fringe width becomes

When a this glass plate of thickness $t$ and refractive index $\mu$ is introduced in one of the paths of the interfering waves then the path increases by $(\mu-1) \mathrm{t}$ and the whole pattern shifts by $$ \mathrm{y}_{0}=\frac{\mathrm{D}}{\mathrm{d}}(\mu-1) \mathrm{t} $$ Shifting is towards the side in which the plate is introduced without any change in fringe width. Therefore, when a glass plate of refractive index $1.5$ is kept in the path of light from one of the slits, only the fringes get shifted but the fringe width remains unchanged.