In a Young's double slit experiment two slits are separated by

(4)

Fringe width $(\beta)=\frac{\lambda D}{d}$

$d=2 \times 10^{-3} \mathrm{~m}$

$\lambda=500 \times 10^{-9} \mathrm{~m}$

$\mathrm{D}=1 \mathrm{~m}$

Now

$\beta=\frac{500 \times 10^{-9} \times 1}{2 \times 10^{-3}}$

$\beta=\frac{5}{2} \times 10^{-4}$

$\beta=2.5 \times 10^{-4}$

$\beta=0.25 \mathrm{~mm}$