The radius of first half period zone is 0.05 cm  and λ=2500 A0 .  The distance between slit and screen is

Radius r=0.05 cm=0.05×10−2 m Wavelength λ=2500 A0 Distance between slit and screen b=?        rn=nbλ ⇒b=r2nλ       =(5×10−4)21×2500×10−10                   =25×10−825×10−8 ∴  b=1 m