1. The problem statement, all variables and given/known data Derive an expression for the Fresnel Zone Plate's focal length 2. Relevant equations The fresnel zone plate has a transmittance function 1/2 (1+cos (ar^2)circ(r/2l) 3. The attempt at a solution Using Eulers formula. this splits into e^(iar^2)+e^(-iar^2) Since the transmittance funciton of a lens is e^(i[itex]\pi[/itex]r^2/[itex]\lambda[/itex]f) The fresnel zone transmittance function is pretty much equivalent to a lens which satisfies a=[itex]\pi[/itex]/[itex]\lambda[/itex]f Hence; the focal length is f= [itex]\pi/[/itex]/[itex]\lambda[/itex]a Is this right?