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

Explanation

Here, `a = 4 mm = 4 xx 10^(-3)m, lambda = 400 nm = 400 xx 10^(-9)m = 4 xx 1^(-7)m`
Ray optics is good approximation upto distance equal to Fresnel's distance `(Z_(F))`
`Z_(F) = (a^(2))/(lambda) = (4 xx 10^(-3))^(2)/(4 xx 10^(-7)) = 40 m`