According to cartesian sign conversion, Focal length of first biconvex lens =f1 Focal length of second biconvex lens =f2 Focal length of third biconcave lens =−f3 ∴ Focal length of combination of first and second lenses is
1
f
=
1
f1
+
1
f2
=
f1+f2
f1f2
As, power of lens, P1=
1
f
=
f1+f2
f1f2
Similarly, power of combination of first and third lenses is P2=
1
f
=
1
f1
−
1
f3
=
f3−f1
f1f3
and for combination of second and third lenses is P3=