In a Young's double slit experiment two slits are separated by 2mm...

$\beta = \dfrac{\lambdaD}{d} = \dfrac{500 \times {10}^{- 9} \times 1}{2 \times {10}^{- 3}}$

$\beta = \dfrac{5}{2} \times {10}^{- 4}m = 2 . 5 \times {10}^{- 1}mm$

$b = 0 . 25mm$