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

Explanation

Fresnel 's distance `(Zf)` is the distance for which the rays optics is a good approxmation . It is given by the relation ,
`Z _(f) =(a^(2))/(lamda)`
where ,
Aperture ,`a=4 mm =4xx10^(-3)m`
Wavelength of light ,`lamda =400 nm = 400 xx10^(-9)` m
` Z_(F)=((4xx10^(-3))^(2))/(400xx10^(-9)) =40 m`
Therefore , the for which the ray optics is a good approxmation is 40 m .