Answer:
Option B
Explanation:
Path difference $\triangle $x is given by $\frac{\lambda\delta}{2\pi}$
[ where, $\lambda$= wave length of light , $\delta$ = phase difference ]
for n= 4th dark fringe
$\delta$ =(2n-1)$\pi$
$\Rightarrow$ $\delta = (8+1)\pi=9\pi$
$\triangle x=\frac{\lambda 9\pi}{2\pi}=\frac{9}{2}\lambda=\frac{9}{2}\times6\times 10^{-7}$
$ 2.7 \times 10^{-6}$ m= $ 2.7 \times 10^{-4}$ cm
