Estimate the distance for which ray optics a good approximation for an aperture of 4 mm and the wavelength is 400 nm. (or) Calculate the distance a beam of light of wavelength 400 nm can travel without significant broadening, if the aperture is 4 mm wide.
$\left(\text{ Hint }:{\mathrm{Z}}_{\mathrm{F}}=\frac{{\mathrm{a}}^2}{\mathrm{\lambda }}\right)$