For what distance is ray optics a good approximation when the aperture is 3 mm wide and the wavelength is 500 nm ?

Q: For what distance is ray optics a good approximation when the aperture is 3 mm wide and the wavelength is 500 nm ?

Sol: For distance Z ≤ Z_F, Ray optics is the good approximate ,

Fresnel distance , $\large Z_F = \frac{a^2}{\lambda} $

$\large Z_F = \frac{(3 \times 10^{-3})^2}{5 \times 10^{-7}} $

Z_F = 18 m