Question

Estimate the distance for which ray optics is good approximately for an aperture of 2mm and wavelength 500 nm.

Answer 1

Ray optics is good approximation upto Fresnel distance,
`Z_(F) = (a^(2))/(lambda) = (4 xx 10^(-3))^(2)/(400 xx 10^(-9)) = (16 xx 10^(-6))/(4 xx 10^(-7)) = 40m`