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

Which of the following is the most precise device for measuring length ? (a) a vernier calliper with 20 divisions on the sliding scale, coinciding with 19 main scale divisions (b) a screw gauge of pitch 1mm and 100 divisions on the circular scale (c ) an optical instrument that can measure length to within a wavelength of light.