Question

power of concave lens is 4.5 D find its focal length

Answer 1

Before finding focal length of concave lens , you should know power of concave lens is negative or positive .
We know, power will be negative , when we considered diverging lens ( concave lens ) and power will be positive , when we considered converging lens .

Here , in question , you considered " concave lens "
Means power should be negative . e.g., P = -4.5D
Now, power is given by
P = 1/focal length ( in m)
⇒P = 100/focal length ( in cm)
⇒ focal length = 100/P cm
Now, put , P = -4.5D
∴ focal length = -100/4.5 cm = -200/9 cm = -22.2 cm

Answer 2

Answer:

Correct option is

D

f=−0.22 m

f=

P

1

=

−4.5

1

=−0.22 m