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

Fresnel’s distance (Z_F) is the distance for which the ray optics is a good approximation. It is given by the relation.

Z_F = a²/λ

Where, Aperture width, a = 4 mm = 4 x 1o^-3 m

Wavelength of light, λ = 400 nm = 400 x 10^-9 m

Z_F = (4 x 1o^-3 m)²/(400 x 10^-9) = 40m

Therefore, the distance for which the ray optics is a good approximation is 40 m.