Given, wavelength of light beam, λ=600nm =600×10−9m Distance between slits, d=1mm=10−3m Distance between slits and screen, D=2m Distance between first dark fringes from the central bright fringe, 2β= ? As we know, fringe width, β=
λD
d
=
600×10−9×2
10−3
=1.2×10−3m Distance between first dark fringes on either side of central bright fringe (2β)=2×β=2×1.2×10−3m ⇒2β=2.4mm