A strong magnetic field is generated by passing a current through a set of windings. This field acts as a convex lens, bringing off axis rays back to focus. Click on 'Draw Rays' opposite to compare the action of an electromagnetic lens with an optical lens.
The image is rotated to a degree that depends on the strength of the lens. After the rays have been drawn, the strength of the lens can also be changed opposite. Focal length can be altered by changing the strength of the current.
Click here for more detail on how magnetic fields affect electrons.