In a Young’s Slit experiment, laser light of 630 nm is used to illuminate two slits of separation 0.20 mm.  Calculate the fringe separation on a screen 3 m away.              

l = ws/D

w = Dl/s = 3.0 m × 630 × 10^-9 m ÷ 0.20 × 10^-3 m (P)

=  0.0095 m (= 9.5 mm) (P)