Question

fringe separation i.e.,fringe width.

Answer 1

$\lambda =630nm$
$d=1.8mm$

$D=80cm$
Fringe width$=B=\frac{D\lambda }{d}$
$=\frac{0.8\times 630\times {10}^{-9}}{1.8\times {10}^{-3}}$
$=2.8\times {10}^{-4}m$