Question

A plane monochromatic light falls normally on a diaphragm with two narrow slits separated by a distance $d=2.5 \, mm$. A fringe pattern is formed on the screen placed at $D=100\, cm$ behind the diaphragm. If one of the slits is covered by a glass plate of thickness $10\, \mu m$, then distance by which these fringes will be shifted will be :

• A. $2 \,mm$
• B. $3\, mm$
• C. $4\, mm$
• D. $5\, mm$

Answer 1

Answer: (A)

If shift of fringes is $x$ then we use
$\frac{x d}{D}=(\mu-1) t$
$\Rightarrow x=\frac{(\mu-1) t D}{d}$
$\Rightarrow x=\frac{(1.5-1) \times 10^{-3} \times 100}{0.25}=0.2\, cm$