
12
2.
Preliminaries: Definitions and Paraxial Optics
Fig. 2.3. Angular magnification. See Eq. (2.2.8) for definition.
Equation (2.2.8) relates the transverse and angular magnification for a pair of
conjugate planes. Rewriting this relation we get
(2.2.9)
nh
tan
u = n'h' \2inu\
which in the paraxial approximation becomes
nhu = n'h'u'. (2.2.10)
If, as is customary, we let H = nhtanu, then Eq. (2.2.9) states that H before
refi-action is the same as H after refi-action. Thus in any optical system containing
any number of refracting (or reflecting) surfaces, H is an invariant. This follows
because the combination n'h'u' for the first surface is