tan
;
,
tan
δ
δ
=
=
=
fh
h
f
if
1
1
 or 
δ =f1
 for small
value of d Thus,
P = 
f1
(9 25)
The SI unit for power of a lens is dioptre (D): 1D = 1m–1 The power of
a lens of focal length of 1 metre is one dioptre