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 le A_(F)`,
ray optics is the good appropriate
Fresnel distance
` Z_(F) = (a^(2))/(lambda) = ((3 xx 10^(-3))^(2))/(5 xx 10^(-7)) = 18 m `