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

Explanation

For distance `Z lt= Z_F` ray optics is the good appropriate Fresnel distance
`Z_F = (a^2)/(lamda) = ((3 xx 10^(-3))^2)/(5 xx 10^(-7)) = 18m`