Question

the focal length of convex lens is 40 cm. its power in dioptre is

Answer 1

p=1/f
=1/40=0.025 dioptre

Answer 2

P1=1f1inmeter
P
1
=
1
f
1
i
n
m
e
t
e
r
=10040
=
100
40
=2.5D
=
2.5
D
P2=−10025
P
2
=
−
100
25
=−4D
=
−
4
D
P=P1+P2
P
=
P
1
+
P
2
=P1+P2=−1.5D