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  4. Young's double slit experiment gives interference fringes of width 0.3 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

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Q. Young's double slit experiment gives interference fringes of width $0.3\, mm$. 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

KCETKCET 2009Wave Optics

Solution:

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 $(\mu-1) t$ and the whole pattern shifts by
$y_{0}=\frac{D}{d}(\mu-1) t$
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.