Fresnel’s distance (ZF) is the distance for which the ray optics is a good approximation. It is given by the relation,
Where,
Aperture width, a = 4 mm = 4 ×10−3 m
Wavelength of light, λ = 400 nm = 400 × 10−9 m
Therefore, the distance for which the ray optics is a good approximation is 40 m.
