The power P of
a lens is defined as the tangent of the angle by which it
converges or diverges a beam of light parallel to the
principal axis falling at unit distance from the optical
centre (Fig 9 18) tan
;
,
tan
δ
δ
=
=
=
fh
h
f
if
1
1
 or 
δ =f1
 for small
value of d