For what distance is ray optics a good approximation when the aperture is 3 mm wide and the wavelength is 500 nm ?

Answer

`18 m`

Explanation

`Z_(F) = ?, lambda = 500 nm = 500 xx 10^(-9)m = 5 xx 10^(-7)m`
`a = 3 mm = 3 xx 10^(-3)m`
`Z_(F) = (a^(2))/(lambda) = ((3 xx 10^(-3))^(2))/(5 xx 10^(-7)) = 18 m`