Given a = 4 mm = 4 × 10-3 m.
λ = 400 nm = 4 × 10-7m
ZF = ?
∴ The minimum distance a beam of light has to travel before its deviation from straight line path becomes significant is called Fresnel distance ZF
∴ ZF = \(\frac{a^2}{λ}\) = \(\frac{16\times10^{-6}}{4\times10^{-7}}\) = 40 m.