Fresnel distance required for a sufficient spreading of central bright fringe, so that diffraction is appreciable $D_F$ = $\frac{a^2}{\lambda}$ = $\frac{(4 \times 10^{-3})^2}{400 \times 10^{-9}}$ = 40 m So, for distance less than 40 m between slit and screen, ray optics is a good approximation as within this distance, the spreading is negligible.