For what distance is ray optics a good approximation when the aperture is 4 mm wide and the wavelength is 500 nm?

Options

A. `20` m
B. `40` m
C. `30` m
D. `50` m

Answer

B

Explanation

Here, `a=4mm=4xx10^(-3)m`
`lambda=400"nm"=400xx10^(-9)m=4xx10^(-7)m`
Ray optics is good approximation upto distances equal to Fresnel.s distance `(Z_(F))`
`Z_(F)=(a^(2))/(lambda)=((4xx10^(-3))^(2))/(4xx10^(-7))=40m`