Estimate the distance for which ray optics is good approximation for an aperture of `4 mm` and wavelength `400 nm`.

Explanation

Here, `a=4`mm`=4xx10^(-3)`m.
`lambda=400` nm `=400xx10^(-9)m=4xx10^(-7)m`.
`:.` Fresnel distance, `Z_(F)=a^(2)/lambda=((4xx10^(-3))^(2))/(4xx10^(-7))=(16xx10^(-6))/(4xx10^(-7))=40 M`.