Given, separation between two slits, d=0.9mm=0.9×10−3m Distance between slits and screen, D=1m The distance between two consecutive dark or bright fringes is given as β (fringe width) and that between central fringe and first dark fringe on either side is
β
2
. It is given that the spacing between second dark fringe and central fringe =β+
β
2
=
3β
2
As we know, β=
λD
d
Hence,
3
2
×
λD
d
=1×10−3m ⇒
3
2
×λ×
1
0.9×10−3
=1×10−3m ⇒λ=
2
3
×10−3×
0.9×10−3
1
=0.6×10−6m ∴ Wavelength of monochromatic source of light, λ=600×10−9m=600nm