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

Explanation

`Z_(F)=(a^(2))/(lambda)=((3xx10^(-3))^(2))/(5xx10^(-7))=18m`
This example shows that even with a small aperture, diffraction spreading can be neglected for rays many metres in length. Thus, ray optics is valid in many common situations.