$($a$)$ Construct the ray$-$path diagram for the objective lens of an electron microscope under conditions of

parallel illumination $($i$.$e$.$ zero beam convergence$).$ In this diagram, be careful to draw the pay paths

accurately, so that you can identify the following features on the diagram: the object plane, the image

plane, the back focal plane, the image distance, the object distance, and the focal length.

$($b$)$ Where would the selected area aperture be situated?

$($c$)$ Define the magnification for this lens.

$($d$)$ Define spherical aberration $($in words or with an additional sketch$).$

$($e$)$ Define chromatic aberration $($in words or with an additional sketch$).$