All electromagnetic lenses
act like 'thin' convex
lenses - their thickness can be ignored for most purposes.
The most important property of a thin lens is its focal
length f. Parallel rays entering the lens are
brought to focus at the distance f shown in the diagram.
Try moving the object (red bar) along the optical axis
noting size position, and orientation of the image. Simple
geometry shows that the magnification M, of the lens 
.
u,v and f are related by the thin lens equation:
Deduce the magnification in terms of u and f. What therefore
is the condition for obtaining high magnification? 
