9 5, the two right-angled triangles A¢B¢F and MPF are
similar (For paraxial rays, MP can be considered to be a straight line
perpendicular to CP ) Therefore,
B A
B F
PM
FP
′
′
′
=
or 
B A
B F
BA
FP
′
′
′
=
 (∵PM = AB)
(9