Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.
Fresnel’s distance (ZF) is the distance for which the ray optics is a good approximation. It is given by the relation,
`Z_r = a^2/lambda`
Aperture width, a = 4 mm = 4 ×10−3 m
Wavelength of light, λ = 400 nm = 400 × 10−9 m
`Z_r = (4xx10^(-3))^2/(400 xx 10^(-9)) = 40 m`
Therefore, the distance for which the ray optics is a good approximation is 40 m.