Light from a sodium lamp (λ = 590 nm) falls on YDSE with d = 0.5 mm, screen D = 1 m. Fringe width:

We have β = λD/d. That is β = (590 × 10⁻⁹)(1)/(5 × 10⁻⁴). That is β = 1.18 × 10⁻³ m. So β = 1.18 mm.