Question

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

• A. 0.3 mm
• B. 0.45 mm
• C. 0.15 mm
• D. zero

Answer 1

Answer: (A)

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.