In an electron microscope, electrons scattered from an object of height \(z_{0}\) are focused by a lens at distance \(D_{0}\) from the object and form an image of height \(z_{1}\) at a distance \(D_{1}\) behind the lens. The aperture of the lens is \(A\). Show by direct calculation that the area in the \(\left(z, p_{z}\right)\) phase plane occupied by electrons leaving the object (and destined to pass through the lens) is the same as the phase area occupied by electrons arriving at the image. Assume that \(z_{0} \ll D_{0}\) and \(z_{1} \ll D_{1}\). (from Mechanics, 3rd edition, by Keith R. Symon.)